Add the check name to the clang-tidy diagnostic output.

Summary:
Pass check names all the way from ClangTidyModule through
ClangTidyCheck and ClangTidyContext to ClangTidyError, and output it in
handleErrors. This allows to find mis-behaving check and disable it easily.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2534