Enable -Wimplicit-fallthrough for ANGLE. Also teach MSVC that ANGLE_CRASH() can't return. Also fix instances of the warning in build configurations where UNREACHABLE() can return (e.g. release without dcheck_always_on or debug). If the UNREACHABLE()s are truly unreachable, this change has no behavior change. Bug: chromium:810767 Change-Id: I68f3587cf3e268c3ef634dce7ae3d70399859d0f Reviewed-on: https://chromium-review.googlesource.com/914842 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nico Weber <thakis@chromium.org> Commit-Queue: Nico Weber <thakis@chromium.org>