Add an AST matcher for CastExpr kind

Summary:
This AST matcher will match a given CastExpr kind.
It's an narrowing matcher on CastExpr.

Reviewers: klimek, alexfh, sbenza, aaron.ballman

Subscribers: Prazek, jroelofs, aaron.ballman, klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D19871

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269460 91177308-0d34-0410-b5e6-96231b3b80d8