This task was originally a part of MDEV-31531, but now is a separate task, because it'll be quite a big change itself.

check_db_name() converts the argument to lower case if lower_case_table_name>0.
It uses my_casedn_str(), which converts the data to lower case in-place. As far as in-place conversion won't be possible after we switch to Unicode-14 casefolding rules (because conversion to lower case can increase the string length in Unicode-14), we need to refactor all uses of check_db_name() not to use in-place casefolding conversion.

The current implementation of:

bool check_db_name(LEX_STRING *db);

should be changed to get a constant argument:

bool check_db_name(const LEX_CSTRING &db);

All old-style check_db_name() use cases should be changed in the way that in case of lower_case_table_names>1 the following sequence is done:

• Convert an existing database name to a new database name in lower case into a NEW memory location.
• Call the NEW version of check_db_name() on the NEW memory location.