Improve clang-tidy diagnostic output and filtering.

Summary:
This patch introduces several improvements to clang-tidy diagnostic;
  1. Make filtering of messages from non-user code more reliable. Output an
     error when it or any of the related notes touches user code. This fixes an
     assertion when an error has a location in a system header, and one of the
     notes relates to user code.
  2. In order for 1. to work, subscribe to the static analyzer diagnostics using
     a custom PathDiagnosticConsumer.
  3. Enable colors on supported terminals.
  4. Output FixItHints.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2714