[Driver] Avoid copies in range-based for loops

[Mordante]

Summary

This avoids new warnings due to D68912 adds -Wrange-loop-analysis to -Wall.

clang/lib/Driver/Driver.cpp

 ///
 /// Looks for file specified by FileName sequentially in directories specified
 /// by Dirs.
 ///
 static bool searchForFile(SmallVectorImpl<char> &FilePath,
                           ArrayRef<std::string> Dirs,
                           StringRef FileName) {
   SmallString<128> WPath;
-  for (const StringRef &Dir : Dirs) {
+  for (const std::string &Dir : Dirs) {
     if (Dir.empty())
       continue;
     WPath.clear();
     llvm::sys::path::append(WPath, Dir, FileName);
     llvm::sys::path::native(WPath);
     if (llvm::sys::fs::is_regular_file(WPath)) {
       FilePath = std::move(WPath);
       return true;

clang/lib/Driver/ToolChain.cpp

void ToolChain::addExternCSystemIncludeIfExists(const ArgList &DriverArgs,
   if (llvm::sys::fs::exists(Path))
     addExternCSystemInclude(DriverArgs, CC1Args, Path);
 }
 
 /// Utility function to add a list of system include directories to CC1.
 /*static*/ void ToolChain::addSystemIncludes(const ArgList &DriverArgs,
                                              ArgStringList &CC1Args,
                                              ArrayRef<StringRef> Paths) {
-  for (const auto Path : Paths) {
+  for (const auto &Path : Paths) {
     CC1Args.push_back("-internal-isystem");
     CC1Args.push_back(DriverArgs.MakeArgString(Path));
   }
 }
 
 void ToolChain::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                              ArgStringList &CC1Args) const {
   // Header search paths should be handled by each of the subclasses.