llvm::None fix

(cherry picked from commit 312bc78)

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -3621,7 +3621,7 @@ static Optional<ActorIsolation> getIsolationFromWrappers(
     return None;
 
   if (!nominal->getParentSourceFile())
-    return None;
+    return llvm::None;
 
   ASTContext &ctx = nominal->getASTContext();
   if (ctx.LangOpts.hasFeature(Feature::DisableActorInferenceFromPropertyWrapperUsage)) {