With the flag --always-use-forwarding-table we could end up turning a
String into a ThinString that had a forwarding index set.
This could happen when a String with a forwarding index is externalized.

Bug: chromium:1337469
Change-Id: Iea05586f61e2b78d83d04d5d2e94c4dca2892c1f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3735164Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81660}