Sema: Allow unavailable decls to witness requirements in more conformances.

In #63898 conformance requirement
typechecking was relaxed to allow unavailable decls to witness conformance
requirements as long as the conforming nominal was also unavailable. However,
only nominals that were directly marked unavailable were accepted. Nominals
that are declared in unavailable scopes should also be allowed to have
unavailable wintesses.

Resolves rdar://107052715

Author: tshortli

lib/Sema/TypeCheckProtocol.cpp

@@ -1701,7 +1701,7 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
       }
 
       if (auto adoptingNominal = DC->getSelfNominalTypeDecl()) {
-        if (adoptingNominal->getAttrs().isUnavailable(getASTContext()))
+        if (adoptingNominal->getSemanticUnavailableAttr())
           return true;
       }
 

test/ModuleInterface/actor_availability.swift

@@ -7,9 +7,6 @@
 
 // REQUIRES: VENDOR=apple
 
-// FIXME: rdar://107052715 temporarily disabled the test; fails on ios simulator
-// REQUIRES: rdar107052715
-
 // CHECK: #if compiler(>=5.3) && $Actors
 // CHECK-NEXT: public actor ActorWithImplicitAvailability {
 public actor ActorWithImplicitAvailability {

test/decl/protocol/req/unavailable.swift

@@ -64,6 +64,14 @@ extension ConformsToP5: P {
   func foo(bar: Foo) { }
 }
 
+@available(*, unavailable)
+enum UnavailableEnum {
+  struct ConformsToP6: P {
+    @available(*, unavailable)
+    func foo(bar: Foo) { }
+  }
+}
+
 // Include message string from @available attribute if provided
 protocol Unavail {
   associatedtype T