Simplify the query flattener so that it does not duplicate the WHERE clause

Simplify the query flattener so that it does not duplicate the WHERE clause
of subquery that is being incorporated into the outer query - copies it
directly.  This is more efficient.  And it also fixes the specific test case
show for ticket [f09fcd17810f65f71789525] but it does not resolve the more
general problem that sqlite3ExprDup() does not correctly duplicate expressions
that contain subqueries with window functions.

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