[libTooling] Add `run` combinator to Stencils.

Summary:
This revision adds `run`, a StencilPart that runs a user-defined function that
computes a result over `MatchFinder::MatchResult`.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67969

llvm-svn: 372936