[clang-tidy] Lift common matchers to utils namespace

Summary:
This patch is lifting matchers used by more than one checkers
to the common namespace.

Reviewers: aaron.ballman, alexfh

Subscribers: aaron.ballman, cfe-commits

Differential Revision: http://reviews.llvm.org/D19841

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@269804 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed