AST: Fix incorrect handling of @_originallyDefinedIn(module: "Swift", ...)

Author: slavapestov

lib/AST/ASTMangler.cpp

@@ -2769,7 +2769,7 @@ void ASTMangler::appendModule(const ModuleDecl *module,
 
   // Try the special 'swift' substitution.
   if (ModName == STDLIB_NAME) {
-    if (useModuleName.empty()) {
+    if (useModuleName.empty() || useModuleName == STDLIB_NAME) {
       appendOperator("s");
     } else if (!RespectOriginallyDefinedIn) {
       appendOperator("s");