Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)

There were a problem since pruning uses the field
for comparison (while evaluate_join_record uses longlong),
resulting in pruning failures when comparing DATE to DATETIME.

Fix was to always comparing DATE vs DATETIME as DATETIME,
by adding ' 00:00:00' to the DATE string.

And adding optimization for comparing with 23:59:59, so that
DATETIME_col > '2001-02-03 23:59:59' ->
TO_DAYS(DATETIME_col) > TO_DAYS('2001-02-03 23:59:59') instead
of '>='.

mysql-test/r/partition_pruning.result:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  Updated result-file
mysql-test/t/partition_pruning.test:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  Added testcases.
sql-common/my_time.c:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  removed duplicate assignment.
sql/item.cc:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  Changed field_is_equal_to_item into field_cmp_to_item, to
  better handling DATE vs DATETIME comparision.
sql/item.h:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  Updated comment
sql/item_timefunc.cc:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  Added optimization (pruning) of DATETIME where time-part is
  23:59:59
sql/opt_range.cc:
  Bug#46362: Endpoint should be set to false for TO_DAYS(DATE)
  
  Using the new stored_field_cmp_to_item for better pruning.

sql-common/my_time.c

@@ -450,9 +450,7 @@ str_to_datetime(const char *str, uint length, MYSQL_TIME *l_time,
     }
   }
 
-  DBUG_RETURN(l_time->time_type=
-              (number_of_fields <= 3 ? MYSQL_TIMESTAMP_DATE :
-                                       MYSQL_TIMESTAMP_DATETIME));
+  DBUG_RETURN(l_time->time_type);
 
 err:
   bzero((char*) l_time, sizeof(*l_time));