improve support for declared_attr returning ORMDescriptor

improve support for declared_attr returning ORMDescriptor

Fixed issue in ORM Annotated Declarative which prevented a
:class:`_orm.declared_attr` with or without
:attr:`_orm.declared_attr.directive` from being used on a mixin which did
not return a :class:`.Mapped` datatype, and instead returned a supplemental
ORM datatype such as :class:`.AssociationProxy`.  The Declarative runtime
would erroneously try to interpret this annotation as needing to be
:class:`.Mapped` and raise an error.

Fixed typing issue where using the :class:`.AssociationProxy` return type
from a :class:`_orm.declared_attr` function was disallowed.

Fixes: #9957
Change-Id: I797c5bbdb3d1e81a04ed21c6558ec349b970476f