After AssertNoTypedSlotsInFreeMemory being a CHECK for some time now to
get more test coverage, turn this into a DCHECK again.

This CL also renames the methods used by the sweeper to clear typed
slots in free memory. It was previously called "invalid slots" but
IMHO that could be a bit misleading, since this isn't about object slot
invalidation (where we also filter slots) but only really about slots in
free memory.

Bug: v8:12760
Change-Id: I8f414be06207460531fa54189b9ef1be85f4ecb6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3679578Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80860}