[cxx-interop] Teach API notes to handle enums with DeclContexts that are extern "C".

Refs #4074

Author: zoecarver

clang/lib/Sema/SemaAPINotes.cpp

@@ -884,12 +884,11 @@ void Sema::ProcessAPINotes(Decl *D) {
 
       return;
     }
-
-    return;
   }
 
   // Enumerators.
-  if (D->getDeclContext()->getRedeclContext()->isFileContext()) {
+  if (D->getDeclContext()->getRedeclContext()->isFileContext() ||
+      D->getDeclContext()->getRedeclContext()->isExternCContext()) {
     if (auto EnumConstant = dyn_cast<EnumConstantDecl>(D)) {
       for (auto Reader : APINotes.findAPINotes(D->getLocation())) {
         auto Info = Reader->lookupEnumConstant(EnumConstant->getName());

clang/test/APINotes/Inputs/Frameworks/CXXInteropKit.framework/Headers/CXXInteropKit.apinotes

@@ -2,4 +2,7 @@
 Name: CxxInterop
 Classes:
 - Name: NSSomeClass
-  SwiftName: SomeClass
+  SwiftName: SomeClass
+Enumerators:
+- Name: SomeClassRed
+  SwiftName: red

clang/test/APINotes/Inputs/Frameworks/CXXInteropKit.framework/Headers/CXXInteropKit.h

@@ -1,3 +1,6 @@
 @interface NSSomeClass
   -(instancetype)init;
-@end
+@end
+
+// Named "SomeClassRed" for ast node filtering in the test.
+enum ColorEnum { SomeClassRed, SomeClassGreen, SomeClassBlue };

clang/test/APINotes/objcxx-swift-name.m

@@ -6,4 +6,8 @@
 
 // CHECK: Dumping NSSomeClass:
 // CHECK-NEXT: ObjCInterfaceDecl {{.+}} imported in CxxInteropKit <undeserialized declarations> NSSomeClass
-// CHECK-NEXT: SwiftNameAttr {{.+}} <<invalid sloc>> "SomeClass"
+// CHECK-NEXT: SwiftNameAttr {{.+}} <<invalid sloc>> "SomeClass"
+
+// CHECK: Dumping SomeClassRed:
+// CHECK-NEXT: EnumConstantDecl {{.+}} imported in CxxInteropKit SomeClassRed 'ColorEnum'
+// CHECK-NEXT: SwiftNameAttr {{.+}} <<invalid sloc>> "red"