[Clang importer] Allow @UIActor as an alias for @mainactor.

We've flip-flopped on the name a bit. Accept both for now.

Author: DougGregor

lib/ClangImporter/ImportDecl.cpp

@@ -8036,9 +8036,10 @@ void ClangImporter::Implementation::importAttributes(
     // __attribute__((swift_attr("attribute")))
     //
     if (auto swiftAttr = dyn_cast<clang::SwiftAttrAttr>(*AI)) {
-      // FIXME: Hard-core @MainActor, because we don't have a point at which to
-      // do name lookup for imported entities.
-      if (swiftAttr->getAttribute() == "@MainActor") {
+      // FIXME: Hard-core @MainActor and @UIActor, because we don't have a
+      // point at which to do name lookup for imported entities.
+      if (swiftAttr->getAttribute() == "@MainActor" ||
+          swiftAttr->getAttribute() == "@UIActor") {
         if (Type mainActorType = getMainActorType()) {
           auto typeExpr = TypeExpr::createImplicit(mainActorType, SwiftContext);
           auto attr = CustomAttr::create(SwiftContext, SourceLoc(), typeExpr);

test/IDE/print_clang_objc_async.swift

@@ -40,6 +40,7 @@ import _Concurrency
 // CHECK-NEXT: @actorIndependent func independentMethod()
 // CHECK-NEXT: @asyncHandler func asyncHandlerMethod()
 // CHECK-NEXT: @MainActor func mainActorMethod()
+// CHECK-NEXT: @MainActor func uiActorMethod()
 // CHECK-NEXT: {{^}}  optional func missingAtAttributeMethod()
 // CHECK-NEXT: {{^[}]$}}
 

test/Inputs/clang-importer-sdk/usr/include/ObjCConcurrency.h

@@ -82,6 +82,7 @@ typedef void (^CompletionHandler)(NSString * _Nullable, NSString * _Nullable_res
 -(void)independentMethod __attribute__((__swift_attr__("@actorIndependent")));
 -(void)asyncHandlerMethod __attribute__((__swift_attr__("@asyncHandler")));
 -(void)mainActorMethod __attribute__((__swift_attr__("@MainActor")));
+-(void)uiActorMethod __attribute__((__swift_attr__("@UIActor")));
 
 @optional
 -(void)missingAtAttributeMethod __attribute__((__swift_attr__("asyncHandler")));