accommodate False conditions for unique / index merge

accommodate False conditions for unique / index merge

Fixed issue in ORM annotated declarative where using
:func:`_orm.mapped_column()` with an :paramref:`_orm.mapped_column.index`
or :paramref:`_orm.mapped_column.unique` setting of False would be
overridden by an incoming ``Annotated`` element that featured that
parameter set to ``True``, even though the immediate
:func:`_orm.mapped_column()` element is more specific and should take
precedence.  The logic to reconcile the booleans has been enhanced to
accommodate a local value of ``False`` as still taking precedence over an
incoming ``True`` value from the annotated element.

Fixes: #11091
Change-Id: I15cda4a0a07a289015c0a09bbe3ca2849956604e