[ObjC] Check entire chain of superclasses to see if class layout is statically known (#81335)

As of now, we only check if a class directly inherits from NSObject to
determine if said class has fixed offsets and can therefore "opt-out"
from the non-fragile ABI for ivars.

However, if an NSObject subclass has fixed offsets, then so must the
subclasses of that subclass, so this allows us to optimize instances of
subclasses of subclasses that inherit from NSObject and so on.

To determine this, we need to find that the compiler can see the
implementation of each intermediate class, as that means it is
statically linked.

Fixes: #81369