Custom DiagnosticConsumer parameter of reformat() + silence diagnostics in unit tests.

Summary:
Added tests for clang-format diagnostics. Added DiagnosticConsumer
argument to clang::format::reformat().

Reviewers: klimek, djasper

Reviewed By: djasper

CC: cfe-commits, thakis, rafael.espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D290