Set up a clang area team private discord channel

Now that the Clang Area Team has grown to five people, it would be helpful for us to have a dedicated discord channel for real time area team communications (coordinating schedules for meetings, reminders to do action items, etc). For right now, this would be a private channel that requires an invite to join (so please invite me, @efriedma-quic, @cor3ntin, @erichkeane, and @shafik) but if we find there are discussions happening which the community would need insight into or participate in, it might be made public.