Replace Item_func::get_arg0_date() to Date/Datetime methods

Item_func has this method:

inline bool get_arg0_date(MYSQL_TIME *ltime, ulonglong fuzzy_date)

{

  DBUG_ASSERT(!(fuzzy_date & TIME_TIME_ONLY));

  Datetime dt(current_thd, args[0], fuzzy_date);

  return (null_value= dt.copy_to_mysql_time(ltime));

}

which now handles DATE and DATETIME data types.

After MDEV-16991, it won't be possible to have this method, because DATE and DATETIME will have different rounding behavior.

Under terms of this fact, the following changes will be done:

1. get_arg0_date() will be removed
2. All get_arg0_date() calls will be replaced to instantiation of Date or Datetime (depending on the data type needed) and accessing their members through various methods.
3. To simplify the code in item_timefunc.cc and to reuse some duplicate code, the following methods will be added into Date and/or Datetime:

• daynr()
• to_seconds()
• check_date()
• dayofyear()
• quarter()
• week()
• yearweek()