With the flag --always-use-string-forwarding-table (only used for
testing), we can have young generation strings in the
StringForwardingTable.
We need to update references to these strings when they are evacuated
during mark compact (previously this was only done after scavenge).

Bug: v8:12877, v8:12007
Change-Id: Ie108add176f71dcdf296bd94bdffa664cb75ae02
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3650719Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80575}