cmd/compile: add support for unsafe.{String,StringData,SliceData}

cmd/compile: add support for unsafe.{String,StringData,SliceData}

For #53003

Change-Id: I13a761daca8b433b271a1feb711c103d9820772d
Reviewed-on: https://go-review.googlesource.com/c/go/+/423774
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>