MDEV-39014: FULL JOIN Phase 2

[DaveGosselin-MariaDB]

In phase 1, FULL [OUTER] JOIN was only supported when simplify_joins()
could rewrite it into an equivalent LEFT, RIGHT, or INNER JOIN based
on NULL-rejecting WHERE predicates.  Queries that could not be
rewritten raised ER_NOT_SUPPORTED_YET.  (Phase 1 was not released.)

This commit removes that restriction by adding proper support for FULL
JOIN by executing a 'LEFT JOIN pass' that emits matched rows and left
null-complemented rows, then a second "null-complement" pass which
rescans the right table to emit null-complement rows that were never
matched.

FULL JOIN supports nested joins on the left of the FULL JOIN,
NATURAL FULL JOIN, semi-joins, CTEs / derived tables (kept
materialized when they participate in a FULL JOIN), prepared
statements, stored procedures, and aggregates.  Examples:

  SELECT * FROM (d1 FULL JOIN d2 ON d1.a = d2.a)
              FULL JOIN t3 ON d1.a = t3.a;

  SELECT * FROM t1 NATURAL FULL JOIN t2;

  SELECT * FROM t1 INNER JOIN t2 FULL JOIN t3 ON t1.a = t3.a;

  PREPARE st FROM
    'SELECT COUNT(*) FROM t1 FULL JOIN t2 ON t1.a = t2.a';

Limitations:
  - The join cache is disabled whenever a FULL JOIN is present, which
    can regress plans for large FULL JOINs compared to the rewritten
    cases.  A follow-up will re-enable it where safe.
  - Statistics and cost estimates for the null-complement pass have
    not been fully implemented; the optimizer may under- or
    over-estimate FULL JOIN costs in plans involving multiple
    FULL JOINs.  Again, a follow-up will optimize the cost calculations.
  - Optimizations for constant tables not fully supported.
  - Nested tables on the right side of a FULL JOIN are not yet supported.