INTO clause can be specified only for the last select of a UNION and it
receives the result of the whole query. But it was wrongly allowed in
non-last selects of a UNION which leads to a confusing query result.

Now INTO allowed only in the last select of a UNION.


mysql-test/t/union.test:
  Added a test case for the bug#23345: Wrongly allowed INTO in a non-last select of a UNION.
mysql-test/r/union.result:
  Added a test case for the bug#23345: Wrongly allowed INTO in a non-last select of a UNION.
sql/sql_yacc.yy:
  Bug#23345: Wrongly allowed INTO in a non-last select of a UNION.
  Now INTO allowed only in the last select of a UNION.