Made the parser to support parenthesis around UNION branches.
 This is done by amending the rules of the parser so it generates the correct
 structure.
 Currently it supports arbitrary subquery/join/parenthesis operations in the 
 EXISTS clause. 
 In the IN/scalar subquery case it will allow adding nested parenthesis only 
 if there is an UNION clause after the parenthesis. Otherwise it will just  
 treat the multiple nested parenthesis as a scalar expression.
 It adds extra lex level for ((SELECT ...) UNION ...) to accommodate for the
 UNION clause.


mysql-test/r/subselect.result:
  Bug#14654 : Cannot select from the same table twice within a UNION statement
   - test cases
mysql-test/t/subselect.test:
  Bug#14654 : Cannot select from the same table twice within a UNION statement
   - test cases
sql/sql_yacc.yy:
  Bug#14654 : Cannot select from the same table twice within a UNION statement
   - shuffle around the rules for the parenthesis in subselect