set _render_for_subquery for legacy set ops

set _render_for_subquery for legacy set ops

Adjusted :meth:`_orm.Query.union` and similar set operations to be
correctly compatible with the new capabilities just added in
:ticket:`6661`, with SQLAlchemy 1.4.19, such that the SELECT statements
rendered as elements of the UNION or other set operation will include
directly mapped columns that are mapped as deferred; this both fixes a
regression involving unions with multiple levels of nesting that would
produce a column mismatch, and also allows the :func:`_orm.undefer` option
to be used at the top level of such a :class:`_orm.Query` without having to
apply the option to each of the elements within the UNION.

Fixes: #6678
Change-Id: Iba97ce7fd8a965499853256fd2eb7f61512db60f