[libTooling] Move Transformer files to their own directory/library.

Summary:
The Transformer library has been growing inside of
lib/Tooling/Refactoring. However, it's not really related to anything else in
that directory. This revision moves all Transformer-related files into their own
include & lib directories.  A followup revision will (temporarily) add
forwarding headers to help any users migrate their code to the new location.

Reviewers: gribozavr

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68637

llvm-svn: 374271