Add QueryableAttribute._impl_uses_objects accessor for AssociationProxy

Add QueryableAttribute._impl_uses_objects accessor for AssociationProxy

Fixed regression where new association proxy system was still not proxying
hybrid attributes when they made use of the ``@hybrid_property.expression``
decorator to return an alternate SQL expression, or when the hybrid
returned an arbitrary :class:`.PropComparator`, at the expression level.
This involved futher generalization of the heuristics used to detect the
type of object being proxied at the level of :class:`.QueryableAttribute`,
to better detect if the descriptor ultimately serves mapped classes or
column expressions.

Fixes: #4690
Change-Id: I5b5300661291c94a23de53bcf92d747701720aa1