The coalesce function returned DATETIME type due to a DATETIME argument, but
since it's not a date/time function it can't return correct int value for
it. Nevertheless Item_datetime_cache was chosen to cache coalesce's result
and that led to a wrong result.

Now Item_datetime_cache is used only for those function that could return
correct int representation of DATETIME values.


mysql-test/r/type_datetime.result:
  Added a test case for the bug#57095.
mysql-test/t/type_datetime.test:
  Added a test case for the bug#57095.
sql/item.cc:
  Bug#57095: Wrongly chosen expression cache type led to a wrong result.
  Now Item_datetime_cache is used only for those function that could return
  correct int representation of DATETIME values.