This doesn't enable the warning yet, but adds V8_FALLTHROUGH annotations
in enough places so that v8 can build with the warning on on my linux box.

Found one real bug
(in effect-control-linearizer.cc,
https://chromium-review.googlesource.com/c/v8/v8/+/850392/3/src/compiler/effect-control-linearizer.cc#825
).

Bug: chromium:812686
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3542550b9c24b545641d0f0fc43f28f2780b0ab3
Reviewed-on: https://chromium-review.googlesource.com/911731Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51322}