Replace a few std::string& with StringRef. NFC.

Patch by Косов Евгений!

llvm-svn: 238774
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index 3f89ea6..4007914 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -199,7 +199,7 @@
 
 /// Stringify - Convert the specified string into a C string, with surrounding
 /// ""'s, and with escaped \ and " characters.
-std::string Lexer::Stringify(const std::string &Str, bool Charify) {
+std::string Lexer::Stringify(StringRef Str, bool Charify) {
   std::string Result = Str;
   char Quote = Charify ? '\'' : '"';
   for (unsigned i = 0, e = Result.size(); i != e; ++i) {
diff --git a/clang/lib/Lex/PTHLexer.cpp b/clang/lib/Lex/PTHLexer.cpp
index af7a153..5f63d35 100644
--- a/clang/lib/Lex/PTHLexer.cpp
+++ b/clang/lib/Lex/PTHLexer.cpp
@@ -431,8 +431,7 @@
   Diags.Report(Diags.getCustomDiagID(DiagnosticsEngine::Error, "%0")) << Msg;
 }
 
-PTHManager *PTHManager::Create(const std::string &file,
-                               DiagnosticsEngine &Diags) {
+PTHManager *PTHManager::Create(StringRef file, DiagnosticsEngine &Diags) {
   // Memory map the PTH file.
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> FileOrErr =
       llvm::MemoryBuffer::getFile(file);