Allow a global-actor-isolated class to inherit from a nonisolated one.

lib/Sema/TypeCheckConcurrency.cpp

@@ -3028,14 +3028,7 @@ static bool checkClassGlobalActorIsolation(
   switch (superIsolation) {
   case ActorIsolation::Unspecified:
   case ActorIsolation::Independent:
-    // Allow ObjC superclasses with unspecified global actors, because we do
-    // not expect for Objective-C classes to have been universally annotated.
-    // FIXME: We might choose to tighten this up in Swift 6.
-    if (superclassDecl->getClangNode())
-      return false;
-
-    // Break out to diagnose the error below.
-    break;
+    return false;
 
   case ActorIsolation::ActorInstance:
     // This is an error that will be diagnosed later. Ignore it here.

test/Concurrency/actor_isolation.swift

@@ -956,7 +956,7 @@ actor Counter {
 class C2 { }
 
 @SomeGlobalActor
-class C3: C2 { } // expected-error{{global actor 'SomeGlobalActor'-isolated class 'C3' has different actor isolation from nonisolated superclass 'C2'}}
+class C3: C2 { } // it's okay to add a global actor to a nonisolated class.
 
 @GenericGlobalActor<U>
 class GenericSuper<U> { }

test/Concurrency/global_actor_inference.swift

@@ -219,7 +219,7 @@ class SuperclassWithGlobalActors {
   func j() { }
 }
 
-@GenericGlobalActor<String> // expected-error@+1{{global actor 'GenericGlobalActor<String>'-isolated class 'SubclassWithGlobalActors' has different actor isolation from nonisolated superclass 'SuperclassWithGlobalActors'}}
+@GenericGlobalActor<String> // it's okay to add a global actor to nonisolated
 class SubclassWithGlobalActors : SuperclassWithGlobalActors {
   override func f() { } // okay: inferred to @GenericGlobalActor<Int>