We can fairly easily detect some resync-needed cases when applying the
overrides operations on a Pointer RNA property.
This should cover all cases where an existing override's ID pointer is
changed in its linked data.
We still have to add code to detect when a not-yet-overridden linked ID
needs to become overridden (because its relations to other data-blocks
changed in a way that requires it).
Part of T83811 & D10649.
534f4e90fd