Now that we have forward references, we can replace the body deferring
mechanism with forward references to the entire pointer.

This ensures that objects are always deserialized with their contents
(aside from themselves maybe holding forward refs), and as a result we
can simplify the CanBeDeferred conditions which encode the constraint
that some objects either need immediately have contents, or cannot be
deferred because their fields are changed temporarily (e.g. backing
store refs).

This also means that objects with length fields (e.g. arrays) will
always have those length fields deserialized when the object is
deserialized, which was not the case when the body could be deferred.
This helps us in the plan to make GC possible during deserialization.

Bug: v8:10815
Change-Id: Ib0e5399b9de6027765691e8cb47410a2ccc15485
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390643Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69760}