Using a std::string instead of a StringRef because the Default case synthesizes a temporary std::string from a Twine. Assigning that into a StringRef causes the StringRef to refer to a temporary, and bad things happen.

This fixes a failing test case on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213265 91177308-0d34-0410-b5e6-96231b3b80d8