MDEV-31606 Refactor check_db_name() to get a const argument

Problem:
Under terms of MDEV-27490, we'll update Unicode version used
to compare identifiers to 14.0.0. Unlike in the old Unicode version,
in the new version a string can grow during lower-case. We cannot
perform check_db_name() inplace any more.

Change summary:

- Allocate memory to store lower-cased identifiers in memory root

- Removing check_db_name() performing both in-place lower-casing and validation
  at the same time. Splitting it into two separate stages:
  * creating a memory-root lower-cased copy of an identifier
    (using new MEM_ROOT functions and Query_arena wrapper methods)
  * performing validation on a constant string
    (using Lex_ident_fs methods)

Implementation details:

- Adding a mysys helper function to allocate lower-cased strings on MEM_ROOT:

    lex_string_casedn_root()

  and a Query_arena wrappers for it:

    make_ident_casedn()
    make_ident_opt_casedn()

- Adding a Query_arena method to perform both MEM_ROOT lower-casing and
  database name validation at the same time:

    to_ident_db_internal_with_error()

  This method is very close to the old (pre-11.3) check_db_name(),
  but performs lower-casing to a newly allocated MEM_ROOT
  memory (instead of performing lower-casing the original string in-place).

- Adding a Table_ident method which additionally handles derived table names:

    to_ident_db_internal_with_error()

- Removing the old check_db_name()

sql/sql_table.cc

@@ -10409,7 +10409,7 @@ bool mysql_alter_table(THD *thd, const LEX_CSTRING *new_db,
    till this point for the alter operation.
   */
   if ((alter_info->flags & ALTER_ADD_FOREIGN_KEY) &&
-      check_fk_parent_table_access(thd, create_info, alter_info, new_db->str))
+      check_fk_parent_table_access(thd, create_info, alter_info, *new_db))
     DBUG_RETURN(true);
 
   /*