implement _post_inspect for AliasedInsp

Fixed issue where using :func:`_orm.with_polymorphic` on a leaf class (a
subclass with no further descendants) or a non-inherited class would fail
with an ``AttributeError`` when used in an ORM statement, due to
:func:`_orm.configure_mappers` not being triggered implicitly. The fix
ensures that :class:`.AliasedInsp` participates in the ``_post_inspect``
hook, triggering mapper configuration during ORM statement compilation.

Fixes: #13319
Change-Id: Ic5910474676be41f8c815dc72c38fca8e20cdeb9