ddl: allow defining auto_random on composite primary key

[bb7133]

Enhancement

For now, TiDB doesn't allow defining AUTO_RANDOM attribute on composite primary keys, even when the key is the clustered index:

mysql> create table log ( id bigint auto_random, b bigint, create_time datetime not null, primary key(id, create_time));
ERROR 8216 (HY000): Invalid auto random: column id is not the integer primary key, or the primary key is nonclustered

Sometimes defining a primary key is useful, for example, to match the requirement of partition keys, so we could relax this restriction.