When a name's hash is a forwarding index, HasHashCode() and hash() now
implicitly retreive the hash from the string forwarding table.

Bug: v8:12957
Change-Id: Ibb278df16b1ae6a54c7cbe757c65d3a5210168e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3827880
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82525}