go/types: underlying type of a type parameter is its constraint interface

go/types: underlying type of a type parameter is its constraint interface

This is a port of CL 359016 from types2 to go/types. Some of the code
around untyped nil differed (because we have to treat untyped nil
differently in go/types for historical reasons).

Updates #47916

Change-Id: Ifc428ed977bf2f4f84cc831f1a3527156940d7b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/364716
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>