Remove old check_db_name() from prepare_db_action()

This task is a part of MDEV-31606. It's to remove the old style check_db_name() from prepare_db_action().

Let's do the following:

• Add a new class Lex_ident_db, to store normalized database names: lower-cased if lower-case-table-name says so, and checked to be a valid database name using Lex_ident_fs::check_db_name(), with asserts in the contructor:

  class Lex_ident_db: public Lex_ident_fs

  {

  public:

  Lex_ident_db()

  :Lex_ident_fs(NULL, 0)

  { }

  Lex_ident_db(const char *str, size_t length)

  :Lex_ident_fs(str, length)

  {

  DBUG_SLOW_ASSERT(ok_for_lower_case_names());

  DBUG_SLOW_ASSERT(!check_db_name());

  }

  };

• Add the following method to class DBNameBuffer:

  Lex_ident_db to_lex_ident_db_with_error() const

  {

  LEX_CSTRING tmp= to_lex_cstring();

  if (Lex_ident_fs(tmp).check_db_name_with_error())

  return Lex_ident_db();

  return Lex_ident_db(tmp.str, tmp.length);

  }

• Remove check_db_name() from prepare_db_action()
• Change the "db" parameter to prepare_db_action() from LEX_CSTRING to Lex_ident_db
• Move creation of Lex_ident_db one level upper, in the switch cases handling SQLCOM_CREATE_DB, SQLCOM_DROP_DB, SQLCOM_ALTER_DB_UPGRADE, SQLCOM_ALTER_DB, with help of the class DBNameBuffer.
• Change the "db" parameter from LEX_CSTRNG to Lex_ident_db in the following function:

  int mysql_create_db(THD *thd, const Lex_ident_db &db, DDL_options_st options,

  const Schema_specification_st *create);

  bool mysql_alter_db(THD *thd, const Lex_ident_db &db,

  const Schema_specification_st *create);

  bool mysql_rm_db(THD *thd, const Lex_ident_db &db, bool if_exists);

  bool mysql_upgrade_db(THD *thd, const Lex_ident_db &old_db);

  so these functions know that the database was already normalized and validated.