Add missing test for newtype'd struct imported as an option set

Author: DougGregor

test/Interop/C/struct/Inputs/StructAsOptionSet.apinotes

@@ -0,0 +1,14 @@
+---
+Name: StructAsOptionSet
+Typedefs:
+- Name: WGPUBufferUsage
+  SwiftConformsTo: Swift.OptionSet
+  SwiftWrapper: struct
+Globals:
+- Name: WGPUBufferUsage_None
+  SwiftName: WGPUBufferUsage.none
+- Name: WGPUBufferUsage_MapRead
+  SwiftName: WGPUBufferUsage.mapRead
+- Name: WGPUBufferUsage_MapWrite
+  SwiftName: WGPUBufferUsage.mapWrite
+

test/Interop/C/struct/Inputs/struct-as-option-set.h

@@ -0,0 +1,6 @@
+#include <stdint.h>
+
+typedef uint32_t WGPUBufferUsage;
+static const WGPUBufferUsage WGPUBufferUsage_None = 0x0000000000000000;
+static const WGPUBufferUsage WGPUBufferUsage_MapRead = 0x0000000000000001;
+static const WGPUBufferUsage WGPUBufferUsage_MapWrite = 0x0000000000000002;

test/Interop/C/struct/typedef_option_set.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs/
+
+import StructAsOptionSet
+
+func takeOptionSet<T: OptionSet>(_: T) { }
+
+func useStructAsOptionSet() {
+  let usage: WGPUBufferUsage = [.mapRead, .mapWrite]
+  takeOptionSet(usage)
+}