top-level is @MainActor(unsafe) below swift 6

Co-authored-by: Doug Gregor <[email protected]>

Author: etcwilde

lib/AST/Decl.cpp

@@ -9090,7 +9090,9 @@ ActorIsolation swift::getActorIsolationOfContext(DeclContext *dc) {
   if (auto *tld = dyn_cast<TopLevelCodeDecl>(dc)) {
     if (dc->isAsyncContext() || dc->getASTContext().LangOpts.WarnConcurrency) {
       if (Type mainActor = dc->getASTContext().getMainActorType())
-        return ActorIsolation::forGlobalActor(mainActor, /*unsafe=*/false);
+        return ActorIsolation::forGlobalActor(
+            mainActor,
+            /*unsafe=*/!dc->getASTContext().isSwiftVersionAtLeast(6));
     }
   }