Fix availability inference for unownedExecutor.

Rather that copying availability directly from `UnownedExecutorRef`, we need
to properly infer availability from both it and the enclosing context.
Otherwise, the synthesized `unownedExecutor` will have an availability that
could conflict with our enclosing declaration.

Fixes rdar://83246377.

Author: DougGregor

lib/Sema/DerivedConformanceActor.cpp

@@ -153,11 +153,14 @@ static ValueDecl *deriveActor_unownedExecutor(DerivedConformance &derived) {
   if (property->getFormalAccess() == AccessLevel::Open)
     property->overwriteAccess(AccessLevel::Public);
 
-  // Clone any @available attributes from UnownedSerialExecutor.
-  // Really, though, the whole actor probably needs to be marked as
-  // unavailable.
-  for (auto attr: executorDecl->getAttrs().getAttributes<AvailableAttr>())
-    property->getAttrs().add(attr->clone(ctx, /*implicit*/true));
+  // Infer availability.
+  SmallVector<const Decl *, 2> asAvailableAs;
+  asAvailableAs.push_back(executorDecl);
+  if (auto enclosingDecl = property->getInnermostDeclWithAvailability())
+    asAvailableAs.push_back(enclosingDecl);
+
+  AvailabilityInference::applyInferredAvailableAttrs(
+      property, asAvailableAs, ctx);
 
   auto getter =
     derived.addGetterToReadOnlyDerivedProperty(property, executorType);

test/Concurrency/concurrency_availability.swift

@@ -11,3 +11,11 @@ actor A { }  // expected-error{{concurrency is only available in}}
 // Allow this without any availability for Historical Reasons.
 public func swift_deletedAsyncMethodError() async {
 }
+
+// Ensure that our synthesis of the actor's unownedExecutor does not cause
+// availability errors.
+@available(macOS 12.0, *)
+struct S {
+  actor A {
+  }
+}