Teach the target parsing framework to directly compute the length of all

Teach the target parsing framework to directly compute the length of all
of its strings when expanding the string literals from the macros, and
push all of the APIs to be StringRef instead of C-string APIs.

This (remarkably) removes a very non-trivial number of strlen calls. It
even deletes code and complexity from one of the primary users -- Clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246374 91177308-0d34-0410-b5e6-96231b3b80d8