Speed up clang-tidy when profiling in on.

Summary:
Speed up clang-tidy when profiling in on.
It makes profiling runs twice as fast by reusing the time samples between the
different actions.
It also joins together the sampling of different matchers of the same check.

Reviewers: alexfh

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D5972