cmd/compile/internal/types2: add unsafe.Add and unsafe.Slice

cmd/compile/internal/types2: add unsafe.Add and unsafe.Slice

This is a port of CL 312212, CL 312591 (except check_test.go), and
CL 312790 to types2.

Updates #19367.
Updates #40481.

Change-Id: I58ba0b0dad157baba3f82c909d5eb1268b931be4
Reviewed-on: https://go-review.googlesource.com/c/go/+/312511
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>