[LibTooling] Add Transformer, a library for source-to-source transformations.

Summary: Adds a basic version of Transformer, a library supporting the concise specification of clang-based source-to-source transformations.  A full discussion of the end goal can be found on the cfe-dev list with subject "[RFC] Easier source-to-source transformations with clang tooling".

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ioeric, ABataev, mgorny, jfb, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59376

llvm-svn: 357768