[libTooling] Introduce the MatchConsumer abstraction

Summary:
This revision introduces a separate (small) library for the `MatchConsumer`
abstraction: computations over AST match results.  This abstraction is central
to the Transformer framework, and there deserves being defined explicitly.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67961

llvm-svn: 372870