Roll back the use of ConstStringRef for now

We might want try a different strategy so hold back on this for the moment, but
fix the off-by-one error in the original function template.

This reverts commit r200190.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200193 91177308-0d34-0410-b5e6-96231b3b80d8