Don't return a pointer to a temporary std::string's c_str.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197157 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid temporary std::string.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197160 91177308-0d34-0410-b5e6-96231b3b80d8