Warn when declared_attr.cascading detected on mapped class

Warn when declared_attr.cascading detected on mapped class

A warning is emitted if the :attr:`.declared_attr.cascading` modifier
is used with a declarative attribute that is itself declared on
a class that is to be mapped, as opposed to a declarative mixin
class or ``__abstract__`` class.  The :attr:`.declared_attr.cascading`
modifier currently only applies to mixin/abstract classes.

Also add a test for @declared_attr.cascading when used on an attribute
on __abstract__.

Change-Id: Ib1b9dbe373e8be1cf24eadfed224a8988b3cd95d
Fixes: #3847