if embedded in a SELECT
            
An ORDER BY clause was bound to the incorrect
(sub-)statement when used in a UNION context.
            
In a query like:
SELECT * FROM a UNION SELECT * FROM b ORDER BY c
the result of SELECT * FROM b is sorted, and then
combined with a.  The correct behaviour is that
the ORDER BY clause should be applied on the
final set.   Similar behaviour was seen on LIMIT
clauses as well.
            
In a UNION statement, there will be a select_lex
object for each of the two selects, and a 
select_lex_unit object that describes the UNION
itself.  Similarly, the same behaviour was also
seen on derived tables.
            
The bug was caused by using a grammar rule for
ORDER BY and LIMIT that bound these elements
to thd->lex->current_select, which points to the
last of the two selects, instead of to the 
fake_select_lex member of the master select_lex_unit
object.


sql/sql_yacc.yy:
  Need to use (opt_)union_order_or_limit to
  bind to the correct select_lex object.