This is a reland of commit 60e9b503

Original change's description:
> cppgc: Consistently treat sentinel pointer as live
>
> Sentinel pointers would be treated as live by the GC (through
> `HandleWeak()` but would be treated as dead when checked explicitly
> through the `LivenessBroker` in e.g. custom callbacks.
>
> Treat sentinel pointers as live consistently across all callsites
> and weak types.
>
> Change-Id: I9a4c096ddac1a111df808f3683325b55e7597eea
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3782800
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Omer Katz <omerkatz@chromium.org>
> Reviewed-by: Anton Bikineev <bikineev@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#81916}

Change-Id: Ie2476345b9ea8406015a3b07bd6880c1159ede08
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3779913Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81975}