Consistently handle clang-tidy check names in ClangTidyError.

Summary:
This patch removes " [check-name]" from the end of
ClangTidyMessage::Message. The " [check-name]" part is only appended when
printing diagnostics on the console. Clang errors are now marked with
"clang-diagnostic-error" check name, remarks and unknown warnings are marked
with "clang-diagnostic-unknown".

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4356