... and rename Untagged to Int32.

Bug: v8:7700
Change-Id: Ib206dba74432b3f1f70fd62006cb6a1222b40ed9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3584114Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80050}