Speed up check by using a recursive visitor.

Summary:
Use a recursive visitor instead of forEachDescendant() matcher.
The latter requires several layers of virtual function calls for each node and
it is more expensive than the visitor.
Benchmark results show improvement of ~6% walltime in clang-tidy.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20597

llvm-svn: 270714