When we cast datetime value to DATE (TIME) type we should throw away its
time (date) part. This was not done properly if CAST() function was used
in datetime expressions.


mysql-test/r/cast.result:
  Added test for bug #6914 "Problems using time()/date() output in
  expressions".
mysql-test/t/cast.test:
  Added test for bug #6914 "Problems using time()/date() output in
  expressions".
sql/item_timefunc.cc:
  Item_time_typecast::get_time()/Item_date_typecast::get_date():
    When we cast datetime value to DATE we should throw away its time part.
    When we cast such value to TIME type we should throw away its date part.