Make it possible to create union main part for a CTE

[sbuerk]

Q	A
Type	bug
Fixed issues	#7318

Summary

Technically this change adopts the solution for adding
the CTE with parts to a SELECT query for UNION query
handling using the WithSQLBuilder in case with parts
has been set to allow creating queries like

WITH
-- CTE with parts
  cte_a AS (SELECT * FROM a_table)

-- CTEmain part
      (SELECT cte_a.*, 'lit1' from cte_a)
UNION (SELECT cte_a.*, 'lit2' from cte_a)

using the union and with api on the same level:

$qb = $connection->createQueryBuilder();

$cte = $qb->sub()
  ->select('*')
  ->from('a_table');

$union1 = $qb->sub()
  ->select('cte_a.*', $qb->expr()->literal('lit1'))
  ->from('cte_a');

$union1 = $qb->sub()
  ->select('cte_a.*', $qb->expr()->literal('lit2'))
  ->from('cte_a');

$qb->with('cte_a', $cte)
  ->union($union1)
  ->addUnion($union2);

This is a valid use-case and supported by databases supporting
common table expressions albeit I could not find that documented
in any documentation and real world use-cases exists and is the
reason why this change has been considered as bugfix.