Change type per review request. Change variable name to fit on a line.

It is OK to use `StringRef` because the method type is `void ModuleDepCollector::addFileDep(StringRef Path)`
so we aren't adding extra unnecessary `StringRef` instances.

Author: vsapsai

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -616,9 +616,8 @@ void ModuleDepCollectorPP::EndOfMainFile() {
   if (!MDC.ScanInstance.getPreprocessorOpts().ImplicitPCHInclude.empty())
     MDC.addFileDep(MDC.ScanInstance.getPreprocessorOpts().ImplicitPCHInclude);
 
-  for (const std::string &Overlay :
-       MDC.ScanInstance.getHeaderSearchOpts().VFSOverlayFiles)
-    MDC.addFileDep(Overlay);
+  for (StringRef VFS : MDC.ScanInstance.getHeaderSearchOpts().VFSOverlayFiles)
+    MDC.addFileDep(VFS);
 
   for (const Module *M :
        MDC.ScanInstance.getPreprocessor().getAffectingClangModules())