[SE-0306] Ban "open" and "required" on actors.

Because actors don't have inheritance, ban "open" and "required", which
don't make sense. We will permit "final" which, although it doesn't
have any semantic impact, is still used to determine whether the ABI
of the actor itself might permit subclassing in the future. This
leaves the door slightly ajar for actor inheritance should we need to
revisit that decision.

Fixes SR-14785 / rdar://79401150.

(cherry picked from commit e595b4f)

Author: DougGregor

include/swift/AST/Decl.h

@@ -7532,7 +7532,8 @@ inline bool Decl::isPotentiallyOverridable() const {
       isa<SubscriptDecl>(this) ||
       isa<FuncDecl>(this) ||
       isa<DestructorDecl>(this)) {
-    return getDeclContext()->getSelfClassDecl();
+    auto classDecl = getDeclContext()->getSelfClassDecl();
+    return classDecl && !classDecl->isActor();
   } else {
     return false;
   }

lib/AST/Decl.cpp

@@ -3345,9 +3345,6 @@ AccessLevel ValueDecl::getFormalAccess() const {
 }
 
 bool ValueDecl::hasOpenAccess(const DeclContext *useDC) const {
-  assert(isa<ClassDecl>(this) || isa<ConstructorDecl>(this) ||
-         isPotentiallyOverridable());
-
   AccessLevel access =
       getAdjustedFormalAccess(this, useDC,
                               /*treatUsableFromInlineAsPublic*/false);

lib/Sema/TypeCheckAttr.cpp

@@ -827,7 +827,9 @@ void AttributeChecker::visitAccessControlAttr(AccessControlAttr *attr) {
   }
 
   if (attr->getAccess() == AccessLevel::Open) {
-    if (!isa<ClassDecl>(D) && !D->isPotentiallyOverridable() &&
+    auto classDecl = dyn_cast<ClassDecl>(D);
+    if (!(classDecl && !classDecl->isActor()) &&
+        !D->isPotentiallyOverridable() &&
         !attr->isInvalid()) {
       diagnose(attr->getLocation(), diag::access_control_open_bad_decl)
         .fixItReplace(attr->getRange(), "public");
@@ -2099,7 +2101,8 @@ void AttributeChecker::visitRequiredAttr(RequiredAttr *attr) {
     return;
   }
   // Only classes can have required constructors.
-  if (parentTy->getClassOrBoundGenericClass()) {
+  if (parentTy->getClassOrBoundGenericClass() &&
+      !parentTy->getClassOrBoundGenericClass()->isActor()) {
     // The constructor must be declared within the class itself.
     // FIXME: Allow an SDK overlay to add a required initializer to a class
     // defined in Objective-C

test/IRGen/async/Inputs/resilient_actor.swift

@@ -1,5 +1,4 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module -enable-experimental-concurrency -enable-library-evolution -emit-module-path=%t/resilient_actor.swiftmodule -module-name=resilient_actor %S/Inputs/resilient_actor.swift
 // RUN: %target-swift-frontend -I %t -emit-ir -enable-experimental-concurrency -enable-library-evolution %s | %IRGenFileCheck %s
 // REQUIRES: concurrency
 
@@ -8,8 +7,6 @@
 //   0x81810050: the same, but using a singleton metadata initialization
 // CHECK-SAME: i32 {{-2122317744|-2122252208}},
 
-import resilient_actor
-
 // CHECK-LABEL: define hidden swiftcc void @"$s13default_actor1ACfD"(%T13default_actor1AC* swiftself %0)
 // CHECK-NOT: ret void
 // CHECK:     call swiftcc void @swift_defaultActor_deallocate(