support selectin_polymorphic w/ no fixed polymorphic_on

support selectin_polymorphic w/ no fixed polymorphic_on

Fixed issue where the :func:`_orm.polymorphic_selectin` loader option would
not work with joined inheritance mappers that don't have a fixed
"polymorphic_on" column.   Additionally added test support for a wider
variety of usage patterns with this construct.

Fixed bug where :func:`_orm.composite` attributes would not work in
conjunction with the :func:`_orm.selectin_polymorphic` loader strategy for
joined table inheritance.

Fixes: #7799
Fixes: #7801
Change-Id: I7cfe32dfe844b188403b39545930c0aee71d0119