Add class DBNameBuffer, split check_db_name() into stages

Under terms of MDEV-31606 we'll split check_db_name() in two separate stages:

• On the first stage, the database name will be converted to lower case if lowercase-table-table says so.
• On the second stage, the database name will actually be validated.

The lower-cased database name is currently created:

• either on THD::mem_root (in many cases)
• or in a fixed size stack buffer (in rare cases, see show_create_db() as an example).

Under terms of this task we'll do the following

• Add a new class Lex_ident_fs, to store identifiers for database objects stored on disk whose case sensitivity depend on lower-case-table-name.
• Extract the validation code from check_db_name() to Lex_ident_fs::check_db_name(). This method will get a constant argument with the database name.
• Add a new class DBNameBuffer to store an optionally lower-cased database name in a stack variable
• Replace check_db_name() in show_create_db(), as well as in other places where it is suitable to use a temporary stack variable, to DBNameBuffer instantiation followed by a Lex_ident_fs::check_db_name() call.

Note, the code which creates a lower-cases database name on THD::mem_root will be modified later, under terms of MDEV-31606.