[gtest] Fix printing of StringRef and SmallString in assert messages.

Summary:
These are detected by gtest as containers, and so previously printed as e.g.
  { '.' (46, 0x2E), 's' (115, 0x73), 'e' (101, 0x65), 'c' (99, 0x63), '0' (48, 0x30) },

gtest itself overloads PrintTo for std::string and friends, we use the same mechanism.

Reviewers: labath

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66520

llvm-svn: 369518