MDEV-34506 2nd execution name resolution problem with pushdown into unions

Statements affected by this bug need all the following to be true 1) a derived table table or view whose specification contains a set operation at the top level. 2) a grouping operator (group by/having) operating on a column alias other than in the first select of the union/intersect 3) an outer condition that will be pushed into all selects in this union/intersect, either into the where or having clause When pushing a condition into all selects of a unit with more than one select, pushdown_cond_for_derived() renames items so we can re-use the condition being pushed. These names need to be saved and reset for correct name resolution on second execution of prepared statements. Reviewed by Igor Babaev (igor@mariadb.com)

Author: mariadb-RexJohnston