Torque allows a `weak` keyword on class field declarations. This keyword
is confusing, because it means two completely different things:

1. This field should be included in the weak fields section, meaning the
   field's offset should be in the range [kStartOfWeakFieldsOffset,
   kEndOfWeakFieldsOffset).
2. If a BodyDescriptor is generated for this class, then this field
   should be visited using *custom* weakness semantics
   (IterateCustomWeakPointers, not IterateMaybeObjectPointers).

I propose the following updated behavior, which I think is a bit more
reasonable:

1. To request that the generated BodyDescriptor use custom weakness
   semantics, use a new annotation @customWeakMarking.
2. The weak fields section includes all fields that can be a Weak<T>
   type, plus those annotated with @customWeakMarking.

These new rules require reordering fields in two classes which didn't
already have all of their strong fields adjacent.

Bug: v8:7793
Change-Id: Ic9d741986afa7fc1be3de044af5cae11a3c64d8c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3261968
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77739}