Distinguish internalized forwarding indices from external forwarding
indices stored in a strings hash.

Bug: v8:12957
Change-Id: Ic01386a3291ac8d618cf4282aa7112e74e1b9169
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3829471Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82513}