[dev.typeparams] go/types: disallow ~T where T is a defined type or an interface

[dev.typeparams] go/types: disallow ~T where T is a defined type or an interface

This is a straightforward port of CL 324570 to go/types.

Change-Id: I1395775a1d21a903a57e0cefc4e240cfa2bb8e97
Reviewed-on: https://go-review.googlesource.com/c/go/+/326684
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>