Try to fix bad Twine usage in r206791 with std::string

I'm pretty sure I was referencing destroyed temporaries here.  I'm open
to suggestions on how to write this better.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206804 91177308-0d34-0410-b5e6-96231b3b80d8