Add always_include_developer_search_paths to mixed_language_library (#1539)

This allows Swift code in the library to depend on XCTest.

If this was passed in `kwargs`, it gets passed along to the internal
`objc_library` targets:
```
~/Development/rules_swift master $ bazel build //examples/apple/mixed_language:MixedAnswer
ERROR: /Users/jschear/Development/rules_swift/examples/apple/mixed_language/BUILD:5:23: //examples/apple/mixed_language:MixedAnswer_headers: no such attribute 'always_include_developer_search_paths' in 'objc_library' rule
ERROR: /Users/jschear/Development/rules_swift/examples/apple/mixed_language/BUILD:5:23: //examples/apple/mixed_language:MixedAnswer_clang: no such attribute 'always_include_developer_search_paths' in 'objc_library' rule
```
We _could_ alternatively `.pop` it off of `kwargs` before passing to
`objc_library`, but tracking which attributes belong to `swift_library`
vs. `obj_library` and which are shared can get a bit complex.

Author: jschear

doc/rules.md

@@ -1066,11 +1066,12 @@ swift_library(
 ## mixed_language_library
 
 <pre>
-mixed_language_library(*, <a href="#mixed_language_library-name">name</a>, <a href="#mixed_language_library-alwayslink">alwayslink</a>, <a href="#mixed_language_library-clang_copts">clang_copts</a>, <a href="#mixed_language_library-clang_defines">clang_defines</a>, <a href="#mixed_language_library-clang_srcs">clang_srcs</a>, <a href="#mixed_language_library-data">data</a>,
-                       <a href="#mixed_language_library-enable_modules">enable_modules</a>, <a href="#mixed_language_library-hdrs">hdrs</a>, <a href="#mixed_language_library-includes">includes</a>, <a href="#mixed_language_library-linkopts">linkopts</a>, <a href="#mixed_language_library-module_map">module_map</a>, <a href="#mixed_language_library-module_name">module_name</a>,
-                       <a href="#mixed_language_library-non_arc_srcs">non_arc_srcs</a>, <a href="#mixed_language_library-package_name">package_name</a>, <a href="#mixed_language_library-private_deps">private_deps</a>, <a href="#mixed_language_library-sdk_dylibs">sdk_dylibs</a>, <a href="#mixed_language_library-sdk_frameworks">sdk_frameworks</a>,
-                       <a href="#mixed_language_library-swift_copts">swift_copts</a>, <a href="#mixed_language_library-swift_defines">swift_defines</a>, <a href="#mixed_language_library-swift_plugins">swift_plugins</a>, <a href="#mixed_language_library-swift_srcs">swift_srcs</a>, <a href="#mixed_language_library-swiftc_inputs">swiftc_inputs</a>,
-                       <a href="#mixed_language_library-textual_hdrs">textual_hdrs</a>, <a href="#mixed_language_library-umbrella_header">umbrella_header</a>, <a href="#mixed_language_library-weak_sdk_frameworks">weak_sdk_frameworks</a>, <a href="#mixed_language_library-deps">deps</a>, <a href="#mixed_language_library-kwargs">**kwargs</a>)
+mixed_language_library(*, <a href="#mixed_language_library-name">name</a>, <a href="#mixed_language_library-always_include_developer_search_paths">always_include_developer_search_paths</a>, <a href="#mixed_language_library-alwayslink">alwayslink</a>, <a href="#mixed_language_library-clang_copts">clang_copts</a>,
+                       <a href="#mixed_language_library-clang_defines">clang_defines</a>, <a href="#mixed_language_library-clang_srcs">clang_srcs</a>, <a href="#mixed_language_library-data">data</a>, <a href="#mixed_language_library-enable_modules">enable_modules</a>, <a href="#mixed_language_library-hdrs">hdrs</a>, <a href="#mixed_language_library-includes">includes</a>, <a href="#mixed_language_library-linkopts">linkopts</a>,
+                       <a href="#mixed_language_library-module_map">module_map</a>, <a href="#mixed_language_library-module_name">module_name</a>, <a href="#mixed_language_library-non_arc_srcs">non_arc_srcs</a>, <a href="#mixed_language_library-package_name">package_name</a>, <a href="#mixed_language_library-private_deps">private_deps</a>, <a href="#mixed_language_library-sdk_dylibs">sdk_dylibs</a>,
+                       <a href="#mixed_language_library-sdk_frameworks">sdk_frameworks</a>, <a href="#mixed_language_library-swift_copts">swift_copts</a>, <a href="#mixed_language_library-swift_defines">swift_defines</a>, <a href="#mixed_language_library-swift_plugins">swift_plugins</a>, <a href="#mixed_language_library-swift_srcs">swift_srcs</a>,
+                       <a href="#mixed_language_library-swiftc_inputs">swiftc_inputs</a>, <a href="#mixed_language_library-textual_hdrs">textual_hdrs</a>, <a href="#mixed_language_library-umbrella_header">umbrella_header</a>, <a href="#mixed_language_library-weak_sdk_frameworks">weak_sdk_frameworks</a>, <a href="#mixed_language_library-deps">deps</a>,
+                       <a href="#mixed_language_library-kwargs">**kwargs</a>)
 </pre>
 
 Creates a mixed language library from a Clang and Swift library target     pair.
@@ -1085,6 +1086,7 @@ Once that is the case, this macro will be deprecated.
 | Name  | Description | Default Value |
 | :------------- | :------------- | :------------- |
 | <a id="mixed_language_library-name"></a>name |  The name of the target.   |  none |
+| <a id="mixed_language_library-always_include_developer_search_paths"></a>always_include_developer_search_paths |  If `True`, the developer framework search paths will be added to the swift compilation command. This enables a Swift module to access `XCTest` without having to mark the target as `testonly = True`.   |  `False` |
 | <a id="mixed_language_library-alwayslink"></a>alwayslink |  If true, any binary that depends (directly or indirectly) on this library will link in all the object files for the files listed in `clang_srcs` and `swift_srcs`, even if some contain no symbols referenced by the binary. This is useful if your code isn't explicitly called by code in the binary; for example, if you rely on runtime checks for protocol conformances added in extensions in the library but do not directly reference any other symbols in the object file that adds that conformance.   |  `False` |
 | <a id="mixed_language_library-clang_copts"></a>clang_copts |  The compiler flags for the clang library. These will only be used for the clang library. If you want them to affect the swift library as well, you need to pass them with `-Xcc` in `swift_copts`.   |  `[]` |
 | <a id="mixed_language_library-clang_defines"></a>clang_defines |  Extra clang `-D` flags to pass to the compiler. They should be in the form `KEY=VALUE` or simply `KEY` and are passed not only to the compiler for this target (as `clang_copts` are) but also to all dependers of this target. Subject to "Make variable" substitution and Bourne shell tokenization.   |  `[]` |

mixed_language/mixed_language_library.bzl

@@ -35,6 +35,7 @@ load("//swift:swift_library.bzl", "swift_library")
 def mixed_language_library(
         *,
         name,
+        always_include_developer_search_paths = False,
         alwayslink = False,
         clang_copts = [],
         clang_defines = [],
@@ -69,6 +70,10 @@ def mixed_language_library(
 
     Args:
         name: The name of the target.
+        always_include_developer_search_paths: If `True`, the developer
+            framework search paths will be added to the swift compilation
+            command. This enables a Swift module to access `XCTest` without
+            having to mark the target as `testonly = True`.
         alwayslink: If true, any binary that depends (directly or indirectly) on
             this library will link in all the object files for the files listed
             in `clang_srcs` and `swift_srcs`, even if some contain no symbols
@@ -308,6 +313,7 @@ a mixed language Swift library, use a clang only library rule like \
         name = swift_library_name,
         srcs = swift_srcs,
         alwayslink = alwayslink,
+        always_include_developer_search_paths = always_include_developer_search_paths,
         aspect_hints = aspect_hints,
         copts = ["-import-underlying-module"] + swift_copts,
         defines = swift_defines,