go/parser: allow parsing aliases with type parameters
We already guard against this in the type checker, and it will
eventually be allowed per the accepted proposal.
Add a placeholder error code for the corresponding type checker error.
Change-Id: I5cc2f1413ecc89ec2094f7178fdb156fb8cc2e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/360235
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>