Remove comment about usage of FATAL, UNREACHABLE and UNIMPLEMENTED,
which was deprecated since https://crrev.com/1410713006.
Also, refactor the FATAL macro and use it for implementing UNREACHABLE
and UNIMPLEMENTED, and in more code. The benefit over printf +
CHECK(false) is that the compiler knows that FATAL will never return.

R=bmeurer@chromium.org

Change-Id: I8c2ab3b4e6edfe8eff5ec6fdf3d92b15d0ed7126
Reviewed-on: https://chromium-review.googlesource.com/832726Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50183}