[libTooling] Move `RewriteRule` abstraction into its own header and impl.

Summary: Move the `RewriteRule` class and related declarations into its own set
of files (header, implementation). Only the `Transformer` class is left in the
Transformer-named files. This change clarifies the distinction between the
`RewriteRule` class, which is essential to the Transformer library, and the
`Transformer` class, which is only one possible `RewriteRule` interpreter
(compare to `TransformerClangTidyCheck`, a clang-tidy based interpreter).

Reviewers: gribozavr

Subscribers: jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68795

llvm-svn: 374558