Reject GADT pattern matches in arrow notation

Tickets #20469 and #20470 show that the current implementation of arrows is not at all up to the task of supporting GADTs: GHC produces ill-scoped Core programs because it doesn't propagate the evidence introduced by a GADT pattern match.

For the time being, this patch rejects GADT pattern matches in arrow notation. Hopefully we will be able to add proper support for GADTs in arrows in the future.