Record old-to-shared references in the C++ write barrier. When
an old-to-shared reference is created, this particular slot will be
atomically inserted into the old-to-new remembered set.

We already stopped clearing the old-to-new-remembered set after a
shared GC, so we already need to be able to handle such slots when
invalidating objects and in the sweeper.

Bug: v8:11708
Change-Id: I1b5854d58f6496228f3a3d9eb7acfd9492f09e68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3557232Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80555}