Since the new space is always empty after a full GC, the old-to-new
remembered set is also always empty after a full GC. This means we can
get rid of the sweeping_slot_set_.

This slot set was used to allow the main thread to insert into the
old-to-new remembered set non-atomically. The sweeping slot set was
owned by the sweeper, which deletes slots in free memory from it. The
main thread would start with an empty old-to-new remembered set. After
sweeping both slot sets are merged again.

The sweeper now needs to behave differently during a GC. When sweeping
a page during full GC, the sweeper needs to delete old-to-new-slots in
free memory.

Outside of the GC the sweeper isn't allowed to remove from the
old-to-new slots anymore. This would race with the main thread that adds
slots to that remembered set while the sweeper is running. However,
there should be no recorded slots in free memory. DCHECKing this is
tricky though, because we would need to synchronize with the main
thread right-trimming objects and at least String::MakeThin only deletes
slots after the map release-store.

Bug: v8:12760
Change-Id: Ic0301851a714e894c3040595f456ab93b5875c81
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3560638Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79713}