fix dangling StringRef

Author: hnrklssn

lib/ClangImporter/ClangImporter.cpp

@@ -2799,9 +2799,10 @@ ClangModuleUnit *ClangImporter::Implementation::getWrapperForModule(
     // Make sure that synthesized Swift code in the clang module wrapper
     // (e.g. _SwiftifyImport macro expansions) can access the same symbols as
     // if it were actually in the clang module
-    StringRef moduleName = isCxxStdModule(I) ? SwiftContext.Id_CxxStdlib.str() :
+    std::string swiftModuleName = isCxxStdModule(I) ?
+      static_cast<std::string>(SwiftContext.Id_CxxStdlib) :
       I->getFullModuleName();
-    ImportPath::Builder importPath(SwiftContext, moduleName, '.');
+    ImportPath::Builder importPath(SwiftContext, swiftModuleName, '.');
     UnloadedImportedModule importedModule(importPath.copyTo(SwiftContext), ImportKind::Module);
     implicitImportInfo.AdditionalUnloadedImports.push_back(importedModule);
   }