Add 3C .clang-tidy files and documentation.

Author: mattmccutchen-cci

clang/docs/checkedc/3C/CONTRIBUTING.md

@@ -86,3 +86,7 @@ in your code. Specifically:
 * Space after the type name, i.e., `Type *K` _not_ `Type* K`.
 
 * Space before and after `:` in iterators, i.e., `for (auto &k : List)`
+
+All files should be formatted with `clang-format` and pass `clang-tidy` ([more
+information](clang-tidy.md)), and nonempty files should have a final newline
+(surprisingly, `clang-format` cannot enforce this).

clang/docs/checkedc/3C/clang-tidy.md

@@ -0,0 +1,38 @@
+# Use of `clang-tidy` on 3C
+
+We're starting to use `clang-tidy` to automatically enforce some of the LLVM
+style guidelines in our code. The main configuration is in
+`clang/lib/3C/.clang-tidy`.
+
+We're currently using version 11.0.0 due to bugs in older versions, so if you
+want to verify that our code is compliant with `clang-tidy`, you'll need to get
+a copy of that version. If it isn't available from your OS distribution, you may
+need to build it from source. This repository is currently based on a
+pre-release of LLVM 9.0.0, so you can't use its version of `clang-tidy`.
+However, Microsoft plans to upgrade it to LLVM 11.0.0 soon, at which point
+you'll have the option to use `clang-tidy` built from this repository.
+
+This file maintains a list of `clang-tidy` problems we've encountered that led
+to warnings we needed to suppress, so those suppressions can refer to the
+corresponding entries in this file.
+
+## `_3C` name prefix
+
+We use `3C` in the names of some program elements. Since an identifier cannot
+begin with a digit, for names that would begin with `3C`, we decided that the
+least evil is to prepend an underscore so the name begins with `_3C`. (A leading
+underscore sometimes suggests that a program element is "private" or "special",
+which is not the case here; this may be misleading.) One alternative we
+considered was to use `ThreeC` at the beginning of a name, but that would be
+inconsistent and would make it more work to search for all occurrences of either
+`3C` or `ThreeC`. And we thought the alternative of using `ThreeC` everywhere
+was just too clunky.
+
+Unfortunately, the current implementation of the [LLVM naming
+guideline](https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly)
+in the `readability-identifier-naming` check does not allow names to begin with
+underscores; in fact, it proposes to remove the underscores, leaving invalid
+identifiers that break compilation. There's no indication that the guideline has
+contemplated our scenario, and we don't know whether its maintainers would allow
+our scenario if they knew about it; we've filed [a
+bug](https://bugs.llvm.org/show_bug.cgi?id=48230) to ask.

clang/include/clang/3C/.clang-tidy

@@ -0,0 +1,19 @@
+# A copy of `clang/lib/3C/.clang-tidy`. We could consider using a symlink if we
+# don't care about Windows.
+Checks: '-*,clang-diagnostic-*,llvm-*,misc-*,-misc-unused-parameters,-misc-non-private-member-variables-in-classes,readability-identifier-naming'
+CheckOptions:
+  - key:             readability-identifier-naming.ClassCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.EnumCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.FunctionCase
+    value:           camelBack
+  - key:             readability-identifier-naming.MemberCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.ParameterCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.UnionCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.VariableCase
+    value:           CamelCase
+

clang/lib/3C/.clang-tidy

@@ -0,0 +1,24 @@
+# Standard clang-tidy options for 3C C++ code. The .clang-tidy files are mostly
+# analogous to the .clang-format files, so the remarks in .clang-format in this
+# directory apply.
+
+# This is currently the same as `/.clang-tidy`. `/clang/.clang-tidy` disables
+# the readability-identifier-naming check, but we mostly comply with it.
+
+Checks: '-*,clang-diagnostic-*,llvm-*,misc-*,-misc-unused-parameters,-misc-non-private-member-variables-in-classes,readability-identifier-naming'
+CheckOptions:
+  - key:             readability-identifier-naming.ClassCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.EnumCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.FunctionCase
+    value:           camelBack
+  - key:             readability-identifier-naming.MemberCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.ParameterCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.UnionCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.VariableCase
+    value:           CamelCase
+

clang/tools/3c/.clang-tidy

@@ -0,0 +1,20 @@
+# A copy of `clang/lib/3C/.clang-tidy`. We could consider using a symlink if we
+# don't care about Windows.
+
+Checks: '-*,clang-diagnostic-*,llvm-*,misc-*,-misc-unused-parameters,-misc-non-private-member-variables-in-classes,readability-identifier-naming'
+CheckOptions:
+  - key:             readability-identifier-naming.ClassCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.EnumCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.FunctionCase
+    value:           camelBack
+  - key:             readability-identifier-naming.MemberCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.ParameterCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.UnionCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.VariableCase
+    value:           CamelCase
+