A prerequisite change for:
- MDEV-8093 sql_yacc.yy: add %type create_field for field_spec and column_def
- MDEV-8094 sql_yacc.yy: get rid of the rules "opt_if_not_exists_table_element"
            and "opt_if_exists_table_element"
- MDEV-8095 Split Create_field