Post-filter clang-tidy diagnostic messages.

Summary:
This patch implements filtering of clang-tidy diagnostic messages by
the check name, so that "clang-tidy -checks=^llvm-" won't output any clang
warnings, for example. This is also helpful to run specific static-analyzer
checks: static analyzer always needs core checks to be enabled, but the user may
be interested only in the checks he asked for.

This patch also exposes warning option names for built-in diagnostics. We need
to have a namespace for these names to avoid collisions and to allow convenient
filtering, so I prefix them with "-W". I'm not sure it's the best thing to do,
and maybe "W" or "clang-diagnostic-" or something like this would be better.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3121

llvm-svn: 204321