A cleanup for MDEV-27896 Wrong result upon `COLLATE latin1_bin CHARACTER SET latin1` on the table or the database level

Changing the error messages in a statement like this:

CREATE DATABASE db1
         COLLATE utf8mb4_bin
         CHARACTER SET utf8mb4
         CHARACTER SET latin1;

from
  COLLATION 'utf8mb4_bin' is not valid for CHARACTER SET 'latin1'

to a more expected:

  Conflicting declarations: 'CHARACTER SET utf8mb4' and 'CHARACTER SET latin1'

In order to do this:
- Adding a new type TYPE_CHARACTER_SET_COLLATE_EXACT into
  Lex_exact_charset_extended_collation_attrs_st

- Removing m_had_charset_exact from its descendant class
  Lex_extended_charset_extended_collation_attrs_st

Additional cleanup:
- Changing methods in Lex_exact_charset_extended_collation_attrs_st
  set_charset(), set_charset_collate_default(), set_charset_collate_binary()
  to get Lex_exact_charset instead CHARSET_INFO as a parameter,
  to guarantee that the argument is only CHARACTER SET and does not have
  any COLLATE clauses yet. This change is not directly related to
  the error message change.