Ensure propagate_attrs available on PropComparator

Ensure propagate_attrs available on PropComparator

Fixed regression caused by just-released performance fix mentioned in #6550
where a query.join() to a relationship could produce an AttributeError if
the query were made against non-ORM structures only, a fairly unusual
calling pattern.

In this fix, since we are no longer going through the production
of ``__clause_element__()`` for Cls.some_relationship, I assumed we
just throw this object away completely but I missed the one little
bit where we might be getting ``_propagate_attrs`` from it.
So we implement ``_propagate_attrs`` on ``PropComparator`` as well,
since this is easy to define.

Fixes: #6558
Change-Id: If781bf844e7e3d3b0841aff1c3668e9d6af9f097