MDEV-36610 Subquery wrongly marked as eliminated by table elimination

[mariadb-RexJohnston]

When equality propagation (build_equal_items()) merges an equality that
contains a subquery, such as "t1.a = (SELECT ...)", with an outer join's
ON equality, it can inject a reference to that subquery into the ON
expression. If the join columns have compatible types the subquery ends
up as the constant of a multiple equality (which Item::walk() skips); if
they differ (e.g. BIGINT vs INT) the field cannot be merged and the
subquery is substituted in as a plain "tbl.col = (SELECT ...)" argument.

In the latter case, if that outer join is removed by table elimination,
mark_as_eliminated() walks the ON expression and flags the shared
Item_subselect as eliminated. The subquery, however, still lives in
another part of the query and has to be executed, tripping
DBUG_ASSERT(!eliminated) in Item_subselect::exec() (and, in release
builds, disabling the subquery cache and hiding it from EXPLAIN).

The surviving reference can be:
 - a WHERE/HAVING/select-list/ORDER/GROUP expression (subquery written
   there and pushed down into the eliminated ON), or
 - the ON expression of an outer join that was not eliminated (subquery
   written in a surviving outer ON and pushed down into an eliminated
   inner one).

Fix: after table elimination, walk the expressions that survive into
execution (WHERE, HAVING, select list, ORDER/GROUP BY and the ON
expressions of outer joins that were not eliminated) and clear the
"eliminated" flag on any subquery still reachable from them.

Because a subquery can also be the constant of a multiple equality, and
Item::walk() does not visit an Item_equal's constant, Item_equal gets an
unmark_as_eliminated_processor() override that descends into its constant
explicitly.

[gemini-code-assist]

Code Review

This pull request addresses two main issues: MDEV-36610, where a subquery was incorrectly marked as eliminated by table elimination due to equality propagation sharing a WHERE-clause subquery into an eliminated outer join's ON expression, and MDEV-39932, ensuring correct ONLY_FULL_GROUP_BY behavior with outer references in correlated subqueries. To fix MDEV-36610, a new unmark_as_eliminated_processor mechanism was introduced to traverse surviving expressions and clear the 'eliminated' flag on reachable subqueries. For MDEV-39932, changes were made to correctly track non-aggregate fields in Item_field::fix_outer_field and Item_sum::check_sum_func. I have no further feedback to provide as there are no review comments.

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