As an escape hatch, add UnsafeConstCast() to still mutate the map
field where necessary.

Drive-by change: Refactor NewPromiseReactionJobTask to avoid unsafe
allocation and map mutations.

Bug: v8:7793
Change-Id: I90e06340c1cf048059b544f1c0a6f730f75d200c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2096675
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66766}