[ClangImporter] Avoid unneeded copy for getExportedModuleName

std::string -> StringRef of long-lived backing storage.

No functionality change.

Author: jrose-apple

include/swift/ClangImporter/ClangModule.h

@@ -62,7 +62,7 @@ class ClangModuleUnit final : public LoadedFile {
   /// Retrieve the "exported" name of the module, which is usually the module
   /// name, but might be the name of the public module through which this
   /// (private) module is re-exported.
-  std::string getExportedModuleName() const;
+  StringRef getExportedModuleName() const;
 
   virtual bool isSystemModule() const override;
 

lib/ClangImporter/ClangImporter.cpp

@@ -3146,7 +3146,7 @@ clang::ASTContext &ClangModuleUnit::getClangASTContext() const {
   return owner.getClangASTContext();
 }
 
-std::string ClangModuleUnit::getExportedModuleName() const {
+StringRef ClangModuleUnit::getExportedModuleName() const {
   if (clangModule && !clangModule->ExportAsModule.empty())
     return clangModule->ExportAsModule;
 

lib/Serialization/Deserialization.cpp

@@ -1149,7 +1149,7 @@ static bool isReExportedToModule(const ValueDecl *value,
       = dyn_cast<ClangModuleUnit>(valueDC->getModuleScopeContext());
   if (!fromClangModule)
     return false;
-  std::string exportedName = fromClangModule->getExportedModuleName();
+  StringRef exportedName = fromClangModule->getExportedModuleName();
 
   auto toClangModule
       = dyn_cast<ClangModuleUnit>(expectedModule->getFiles().front());

lib/Serialization/ModuleFile.cpp

@@ -1847,11 +1847,9 @@ void ModuleFile::loadExtensions(NominalTypeDecl *nominal) {
 
     // If the originating module is a private module whose interface is
     // re-exported via public module, check the name of the public module.
-    std::string exportedModuleName;
     if (auto clangModuleUnit =
             dyn_cast<ClangModuleUnit>(parentModule->getFiles().front())) {
-      exportedModuleName = clangModuleUnit->getExportedModuleName();
-      moduleName = exportedModuleName;
+      moduleName = clangModuleUnit->getExportedModuleName();
     }
 
     for (auto item : *iter) {