Torque currently generates constants like kStartOfWeakFieldsOffset and
kEndOfStrongFieldsOffset, which can be used when writing custom
BodyDescriptors. However, these offsets have some potentially confusing
behaviors:

* They don't take inheritance into account and describe only the fields
  defined by the current class itself, so there might be (for example)
  strong fields before kStartOfStrongFieldsOffset if they were defined
  by a superclass.
* kStartOfWeakFieldsOffset points to the first field defined in Torque
  using the keyword `weak`, which indicates fields with *custom*
  weakness semantics (those that should be visited with
  IterateCustomWeakPointers), not those that may contain standard weak
  pointers (visited with IterateMaybeWeakPointers). (As a follow-up, I'd
  like to also rename `weak` to `@customWeak`.)

Given that these constants have very low usage and somewhat bizarre
semantics, I propose that we remove them. This change does so, and
updates the existing usages to either define the required constants
directly in C++ or not use them. I know that defining these constants in
C++ is more brittle, but I think that brittle and clear is better than
automatic and incomprehensible.

Bug: v8:7793
Change-Id: I87f8c85ccae4027f61ac73d4e7e4e2820e92003b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3199731Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#77411}