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}