[SourceManager] Avoid copying SLocEntry in computeMacroArgsCache

Follow-up to e787022

Differential Revision: https://reviews.llvm.org/D86230

Author: jkorous-apple

clang/lib/Basic/SourceManager.cpp

@@ -1791,7 +1791,7 @@ void SourceManager::computeMacroArgsCache(MacroArgsMap &MacroArgsCache,
     if (Invalid)
       return;
     if (Entry.isFile()) {
-      auto File = Entry.getFile();
+      auto& File = Entry.getFile();
       if (File.getFileCharacteristic() == C_User_ModuleMap ||
           File.getFileCharacteristic() == C_System_ModuleMap)
         continue;