and Order By
      
When having a UNION statement in a subquery, with no
referenced tables (or only a reference to the virtual
table 'dual'), the UNION did not allow an ORDER BY clause.
      
      i.e:
          SELECT(SELECT 1 AS a UNION 
                 SELECT 0 AS a 
                 ORDER BY a) AS b  or
          SELECT(SELECT 1 AS a FROM dual UNION 
                 SELECT 0 as a 
                 ORDER BY a) AS b
      
      
In addition, an ORDER BY / LIMIT clause was not accepted
in subqueries even for single SELECT statements with no 
referenced tables (or with 'dual' as table reference)
      
   i.e: 
      SELECT(SELECT 1 AS a ORDER BY a) AS b  or
      SELECT(SELECT 1 AS a FROM dual ORDER BY a) AS b
      
The fix was to allow an optional ORDER BY/LIMIT clause to
the grammar for these cases.
      
See also: Bug#57986