MDEV-32026 lowercase_table2.test failures in 11.3

Also fixes MDEV-32025 Crashes in MDL_key::mdl_key_init with lower-case-table-names=2

Change overview:
- In changes made in MDEV-31948, MDEV-31982 the code path
  which originaly worked only in case of lower-case-table-names==1
  also started to work in case of lower-case-table-names==2 in a mistake.

  Restoring the original check_db_name() compatible behavior
  (but without re-using check_db_name() itself).
- MDEV-31978 erroneously added a wrong DBUG_ASSERT. Removing.

Details:

- In mysql_change_db() the database name should be lower-cased only
  in case of lower_case_table_names==1. It should not be lower-cased
  for lower_case_table_names==2. The problem was caused by MDEV-31948.
  The new code version restored the pre-MDEV-31948 behavior, which
  used check_db_name() behavior.

- Passing lower_case_table_names==1 instead of just lower_case_table_names
  to the "casedn" parameter to DBNameBuffer constructor in sql_parse.cc
  The database name should not be lower-cased for lower_case_table_names==2.
  This restores pre-MDEV-31982 behavioir which used check_db_name() here.

- Adding a new data type Lex_ident_db_normalized, it stores database
  names which are both checked and normalized to lower case
  in case lower_case_table_names==1 and lower_case_table_names==2.

- Changing the data type for the "db" parameter to Lex_ident_db_normalized in
  lock_schema_name(), lock_db_routines(), find_db_tables_and_rm_known_files().

  This is to avoid incorrectly passing a non-normalized name in the future.

- Restoring the database name normalization in mysql_create_db_internal()
  and mysql_rm_db_internal() before calling lock_schema_name().
  The problem was caused MDEV-31982.

- Adding database name normalization in mysql_alter_db_internal()
  and mysql_upgrade_db(). This fixes MDEV-32026.

- Removing a wrong assert in Create_sp_func::create_with_db() was incorrect:

    DBUG_ASSERT(Lex_ident_fs(*db).ok_for_lower_case_names());

  The database name comes to here checked, but not normalized
  to lower case with lower-case-table-names=2.
  The assert was erroneously added by MDEV-31978.

- Recording lowercase_tables2.results and lowercase_tables4.results
  according to
    MDEV-29446 Change SHOW CREATE TABLE to display default collations
  These tests are skipped on buildbot on all platforms, so this change
  was forgotten in the patch for MDEV-29446.

sql/sql_show.cc

@@ -4598,7 +4598,7 @@ make_table_name_list(THD *thd, Dynamic_array<LEX_CSTRING*> *table_names,
     return (schema_tables_add(thd, table_names,
                               lookup_field_vals->table_value.str));
 
-  DBNameBuffer dbbuf(*db_name, lower_case_table_names);
+  const DBNameBuffer dbbuf(*db_name, lower_case_table_names == 1);
   const Lex_ident_db dbnorm= dbbuf.to_lex_ident_db();
   if (!dbnorm.str)
     return 0; // Impossible TABLE_SCHEMA name