Clang-tidy: don't create ASTConsumers not needed for the set of checks we perform.

Summary:
This doesn't have any significant effect on the performance, but it
looks like a good thing to do.

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2763

llvm-svn: 201340