[clang] Replace SmallStr.str().str() with std::string conversion operator.

Use the std::string conversion operator introduced in
d7049213d0fcda691c9e79f9b41e357198d99738.
diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index ff9272e..73c02d7 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -159,7 +159,7 @@
     llvm::sys::fs::make_absolute(Result);
     llvm::sys::path::append(Result, ModuleName + ".pcm");
     if (getFileMgr().getFile(Result.str()))
-      return Result.str().str();
+      return std::string(Result);
   }
   return {};
 }