This is a reland of commit d00c0405

Changes since revert: Use AsAtomicTagged instead of
base::AsAtomicPointer to store a hash value in the forwarding table.

Original change's description:
> [strings] Support shared external strings>
>
> With this CL shared strings can be externalized and external strings can
> be shared.
> The StringForwardingTable is used to delay the real transition to the
> next full GC. On the API side strings marked for externalization will
> look like externalized strings.
>
> Bug: v8:12957
> Change-Id: I53b6509129bc5679c06bdf99421bdb41ea5d9082
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3849643
> Reviewed-by: Shu-yu Guo <syg@chromium.org>
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Commit-Queue: Patrick Thier <pthier@chromium.org>
> Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
> Reviewed-by: Victor Gomes <victorgomes@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#82966}

Bug: v8:12957
Change-Id: I17715e927e4339240a6aa12a3c4a3c2ea50eb567
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3871211
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83018}