The per-module caches for subtype relations and type equivalences are
accessed from several background compile jobs, so these accesses must
be guarded by a lock.
This issue was found by our TSan bots and caused the following revert:
https://chromium-review.googlesource.com/c/v8/v8/+/2270734

Bug: v8:7748
Change-Id: I0322972f8f72ca8aff3538bf3f78d4329e5f3a44
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2272564
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68574}