propagate _scalar_type() for SelectStatementGrouping

propagate _scalar_type() for SelectStatementGrouping

Fixed issue where using the :meth:`.ColumnOperators.in_` operator with a
nested :class:`.CompoundSelect` statement (e.g. an ``INTERSECT`` of
``UNION`` queries) would raise a :class:`NotImplementedError` when the
nested compound select was the first argument to the outer compound select.
The ``_scalar_type()`` internal method now properly handles nested compound
selects.

Fixes: #12987
Change-Id: I6aa1b38863588d371bbac74b3531b99ccd5fcaec