Added test cases for bug #12625.
sql_select.cc:
  Fixed bug #12625.
  Fixed invalid removal of constant items from the DISTINCT
  list in the function create_distinct_group.


sql/sql_select.cc:
  Fixed bug #12625.
mysql-test/r/distinct.result:
  Added test cases for bug #12625.
mysql-test/t/distinct.test:
  Added test cases for bug #12625.