... or sometimes by FATAL(...) to give a better error message.
The benefit of UNREACHABLE() over CHECK(false) is that the compiler
knows that this macro will never return, hence we can omit the return
of a dummy value afterwards.

R=neis@chromium.org

Change-Id: I14e6a4f1d75f1338f481bd1520d841fd383d6202
Reviewed-on: https://chromium-review.googlesource.com/832431Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50214}