unittests: explicit stringify StringRefs for conversion

When clang is built outside of the LLVM tree (against a corresponding version),
there is no definition providing for operator<<(std::ostream &, StringRef) which
is required for the assertion routines in google-test tests.  Avoid the
compilation failure by explicitly stringifying the StringRef prior to use.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200096 91177308-0d34-0410-b5e6-96231b3b80d8