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)