[Concurrency] Allow actor classes to inherit from NSObject.

NSObject is guaranteed to have no state and no Swift vtable, and is
necessary for Swift classes to implement the NSObject protocol. Allow
it (and only it) as the superclass of an actor class, so that actor
classes can be exposed to Objective-C.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -180,6 +180,14 @@ bool IsActorRequest::evaluate(
     if (superclassDecl->isActor())
       return true;
 
+    // The superclass is 'NSObject', which is known to have no state and no
+    // superclass.
+    if (superclassDecl->hasClangNode() &&
+        superclassDecl->getName().is("NSObject") &&
+        superclassDecl->getModuleContext()->getName().is("ObjectiveC") &&
+        actorAttr != nullptr)
+      return true;
+
     // This class cannot be an actor; complain if the 'actor' modifier was
     // provided.
     if (actorAttr) {

test/attr/attr_objc_async.swift

@@ -46,3 +46,6 @@ actor class MyActor {
   // CHECK: @objc @actorIndependent func synchronousGood()
   @objc @actorIndependent func synchronousGood() { }
 }
+
+// CHECK: @objc actor class MyObjCActor
+@objc actor class MyObjCActor: NSObject { }