Remove the use of STRING_ITEM from Item_func_date_format::fix_length_and_dec()

We'll be replacing data type specific constants STRING_ITEM, INT_ITEM, REAL_ITEM, DECIMAL_ITEM, DATE_ITEM to LITERAL_ITEM soon (see MDEV-14630).

Under terms of this task we'll remove usage of STRING_ITEM in Item_func_date_format::fix_length_and_dec() by replacing this condition:

  if (arg1->type() == STRING_ITEM)

to

  StringBuffer<STRING_BUFFER_USUAL_SIZE> buffer;

  String *str;

  if (args[1]->basic_const_item() &&

      (str= args[1]->val_str(&buffer)))

The main point in this condition is to catch constants (literals and bound Item_param instances), so calling val_str() can be done directly in fix_length_and_dec() to calculate the result max_length more precisely.

As a good side effect, this change will allow constants of non-string types, to calculate max_length more precisely.