go/types,types2: add support for unsafe.{String,StringData,SliceData}

go/types,types2: add support for unsafe.{String,StringData,SliceData}

For #53003
Change-Id: Id3125268523fed855ffac20cde6128010e3513f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/423754
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>