Disable single-inheritance critera on the outside of UNION

Disable single-inheritance critera on the outside of UNION

Fixed bug related to :ticket:`3177`, where a UNION or other set operation
emitted by a :class:`.Query` would apply "single-inheritance" criteria
to the outside of the union (also referencing the wrong selectable),
even though this criteria is now expected to
be already present on the inside subqueries.  The single-inheritance
criteria is now omitted once union() or another set operation is
called against :class:`.Query` in the same way as :meth:`.Query.from_self`.

Change-Id: I0fd1331c7ba85a758a1c15e06c271914f2c717f3
Fixes: #3856