[clang-tidy] Cleanup some ast-matchers and lift some to utils.

Summary:
Little cleanup to lift-out and to remove some frequently used
ast-matchers.

Some of theses matchers are candidates to be lifted to ASTMatchers.h.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19200

llvm-svn: 267003