Remove the use of STRING_ITEM from the parser

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 STRING_ITEM from:

• The code creating concatenated string literals:

  Item *THD::make_string_literal_concat(Item *item, const LEX_CSTRING &str)

  Instead of testing item->type() for STRING_ITEM, we'll add a virtual method:

  virtual Item_basic_constant *Item_basic_constant::make_string_literal_concat(THD *thd, const LEX_CSTRING *)

  with implementations for Item_null and Item_string, and a default implementation for Item.

• The rule in the bison grammar that parses ODBC style literals:

  | '{' ident expr '}'

  Instead of testing $3->type() for STRING_ITEM, we'll add a virtual method:

  virtual Item *Item::make_odbc_literal(THD *thd, const LEX_CSTRING *typestr);

  with an implementation for Item_string, and a default implementation for Item.