Eliminate support for @mainactor(unsafe) Clang attribute.

The `@MainActor(unsafe)` attribute could be provided for C declarations
via the Clang `swift_attr` attribute. However, this facility was never
used outside of tests, and has been superceded by `@MainActor` with the
inferred `@_predatesConcurrency`.

Author: DougGregor

lib/ClangImporter/ImportDecl.cpp

@@ -8392,17 +8392,13 @@ SourceFile &ClangImporter::Implementation::getClangSwiftAttrSourceFile(
   return *sourceFile;
 }
 
-Optional<bool> swift::importer::isMainActorAttr(
-    ASTContext &ctx, const clang::SwiftAttrAttr *swiftAttr) {
+bool swift::importer::isMainActorAttr(const clang::SwiftAttrAttr *swiftAttr) {
   if (swiftAttr->getAttribute() == "@MainActor" ||
-      swiftAttr->getAttribute() == "@MainActor(unsafe)" ||
       swiftAttr->getAttribute() == "@UIActor") {
-    bool isUnsafe = swiftAttr->getAttribute() == "@MainActor(unsafe)" ||
-        !ctx.LangOpts.isSwiftVersionAtLeast(6);
-    return isUnsafe;
+    return true;
   }
 
-  return None;
+  return false;
 }
 
 void
@@ -8430,9 +8426,7 @@ ClangImporter::Implementation::importSwiftAttrAttributes(Decl *MappedDecl) {
   for (auto swiftAttr : ClangDecl->specific_attrs<clang::SwiftAttrAttr>()) {
     // FIXME: Hard-code @MainActor and @UIActor, because we don't have a
     // point at which to do name lookup for imported entities.
-    if (auto isMainActor = isMainActorAttr(SwiftContext, swiftAttr)) {
-      bool isUnsafe = *isMainActor;
-
+    if (isMainActorAttr(swiftAttr)) {
       if (SeenMainActorAttr) {
         // Cannot add main actor annotation twice. We'll keep the first
         // one and raise a warning about the duplicate.
@@ -8446,7 +8440,6 @@ ClangImporter::Implementation::importSwiftAttrAttributes(Decl *MappedDecl) {
       if (Type mainActorType = SwiftContext.getMainActorType()) {
         auto typeExpr = TypeExpr::createImplicit(mainActorType, SwiftContext);
         auto attr = CustomAttr::create(SwiftContext, SourceLoc(), typeExpr);
-        attr->setArgIsUnsafe(isUnsafe);
         MappedDecl->getAttrs().add(attr);
         SeenMainActorAttr = swiftAttr;
       }

lib/ClangImporter/ImportType.cpp

@@ -1726,7 +1726,7 @@ Type ClangImporter::Implementation::applyParamAttributes(
       continue;
 
     // Map the main-actor attribute.
-    if (isMainActorAttr(SwiftContext, swiftAttr)) {
+    if (isMainActorAttr(swiftAttr)) {
       if (Type mainActor = SwiftContext.getMainActorType()) {
         type = applyToFunctionType(type, [&](ASTExtInfo extInfo) {
           return extInfo.withGlobalActor(mainActor);

lib/ClangImporter/ImporterImpl.h

@@ -1663,11 +1663,7 @@ class SwiftNameLookupExtension : public clang::ModuleFileExtension {
 
 /// Determines whether the given swift_attr attribute describes the main
 /// actor.
-///
-/// \returns None if this is not a main-actor attribute, and a Boolean
-/// indicating whether (unsafe) was provided in the attribute otherwise.
-Optional<bool> isMainActorAttr(
-    ASTContext &ctx, const clang::SwiftAttrAttr *swiftAttr);
+bool isMainActorAttr(const clang::SwiftAttrAttr *swiftAttr);
 
 }
 }

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

@@ -178,7 +178,7 @@ typedef void ( ^ObjCErrorHandler )( NSError * _Nullable inError );
 #define MAGIC_NUMBER 42
 
 
-__attribute__((__swift_attr__("@MainActor(unsafe)")))
+__attribute__((__swift_attr__("@MainActor")))
 @interface NXView : NSObject
 -(void)onDisplay;
 @end
@@ -195,7 +195,7 @@ void doSomethingConcurrently(__attribute__((noescape)) SENDABLE void (^block)(vo
 void doSomethingConcurrentlyButUnsafe(__attribute__((noescape)) __attribute__((swift_attr("@Sendable"))) void (^block)(void));
 
 
-MAIN_ACTOR MAIN_ACTOR __attribute__((__swift_attr__("@MainActor(unsafe)"))) @protocol TripleMainActor
+MAIN_ACTOR MAIN_ACTOR __attribute__((__swift_attr__("@MainActor"))) @protocol TripleMainActor
 @end
 
 @protocol ProtocolWithAsync