ThinStrings are essentially a pointer to an InternalizedString. Read
them concurrently in places where we read InternalizedStrings.

Bug: v8:7790, v8:11791
Change-Id: I3be4dd27336f58706c9c57d5042f96cb8f56bcaa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2905608
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74785}