The GC might promote an in-place internalizable string from new space
directly into the shared heap. This means that the GC might need to
create OLD_TO_SHARED slots when updating OLD_TO_NEW slots.

This CL implements this both for minor and full GCs.

Bug: v8:11708
Change-Id: I6102b9024d1dd5dd602d654b006ea5897ab5baa6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3804604
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82298}