go/types: implement unsafe.Add and unsafe.Slice

Updates #19367.
Updates #40481.

Change-Id: Id2b2d2e3e716f91f0dd9e5102689a1ba90a819e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/312213


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>