warn for other mapper property objects assigned twice

warn for other mapper property objects assigned twice

was already in place for columns via other means

A warning is emitted when an ORM :func:`_orm.relationship` and other
:class:`.MapperProperty` objects are assigned to two different class
attributes at once; only one of the attributes will be mapped.  A warning
for this condition was already in place for :class:`_schema.Column` and
:class:`_orm.mapped_column` objects.

Fixes: #3532
Change-Id: Ib8057bdf229aa92137f9b8f61b26d4008181ead6