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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172399 91177308-0d34-0410-b5e6-96231b3b80d8