In addition change DCHECKs to CHECKs in StringForwardingTable.
The added CHECKs hopefully make it easier to reason about crashes on
canary.

Bug: chromium:1336516
Change-Id: I30bbabbc2a9186eaeac42c2963e7ae8dbb9fb527
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3707103Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81194}