[gtest] Add PrintTo overload for StringRef.

Summary:
It was printed using code for generic containers before, resulting in
unreadable output.

Reviewers: sammccall, labath

Reviewed By: sammccall, labath

Subscribers: labath, zturner, llvm-commits

Differential Revision: https://reviews.llvm.org/D43330

llvm-svn: 326092