We clear during sweep so that we are guaranteed the in-construction bit
of newly allocated objects is always 0. The lock sweeping uses for
synchronization assures no data races between clearing and concurrent
marking.

The only exception to that is debug builds that zap on sweep and clear
on allocation. This makes it so that dangling references will most
likely crash in debug builds.

Bug: chromium:1056170
Change-Id: I12597ef76629ec50c6bfc39dc21b68243c4160ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2438530
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70283}