Roll back the ConstStringRef change for now

There are a couple of interesting things here that we want to check over
(particularly the expecting asserts in StringRef) and get right for general use
in ADT so hold back on this one. For clang we have a workable templated
solution to use in the meanwhile.

This reverts commit r200187.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200194 91177308-0d34-0410-b5e6-96231b3b80d8