Includes the new sheriff email alias, and also fixes to the Clang
version detection. The new version checks the CopyBara mirror to find
the real clang rev ANGLE ends up with.

Bug: angleproject:5926
Change-Id: Ie70319feeb5eefbfba8a6b6d2d57110273d6c3e5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2872070
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>