The bug caused wrong result sets for union constructs of the form
(SELECT ... ORDER BY order_list1 [LIMIT n]) ORDER BY order_list2.
For such queries order lists were concatenated and limit clause was
completely neglected. 


mysql-test/r/order_by.result:
  Added a test case for bug #18767.
mysql-test/t/order_by.test:
  Added a test case for bug #18767.
sql/sql_lex.h:
  Fixed bug #18767.
  Placed the code the created a fake SELECT_LEX into a separate function.
sql/sql_parse.cc:
  Fixed bug #18767.
  Placed the code the created a fake SELECT_LEX into a separate function.
sql/sql_select.cc:
  Fixed bug #18767.
  Changed the condition on which a SELECT is treated as part of a UNION.
  The SELECT in 
  (SELECT ... ORDER BY order_list1 [LIMIT n]) ORDER BY order_list2 
  now is handled in the same way as the first SELECT in a UNION
  sequence.
sql/sql_union.cc:
  Fixed bug #18767.
  Changed the condition at which a SELECT is treated as part of a UNION.
  The SELECT in 
  (SELECT ... ORDER BY order_list1 [LIMIT n]) ORDER BY order_list2 
  now is handled in the same way as the first SELECT in a UNION
  sequence.
sql/sql_yacc.yy:
  Fixed bug #18767.
  Changed the condition at which a SELECT is treated as part of a UNION.
  The SELECT in 
  (SELECT ... ORDER BY order_list1 [LIMIT n]) ORDER BY order_list2 
  now is handled in the same way as the first SELECT in a UNION
  sequence. In the same way is handled the SELECT in
  (SELECT ... LIMIT n) ORDER BY order list.
  Yet if there is neither ORDER BY nor LIMIT in the single-select
  union construct
  (SELECT ...) ORDER BY order_list
  then it is still handled as simple select with an order clause.