[dev.typeparams] go/types: adjust unsafe.Alignof/Offsetof/Sizeof

This is a port of CL 335413 to go/types, adjusted for the parsing API of
go/parser.

Change-Id: Ie6836add7d027aaf5d6d3dae65222b1d15bd7558
Reviewed-on: https://go-review.googlesource.com/c/go/+/339649


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>