go/types, cmd/compile/internal/types2: use regular type printing for unsafe.Pointer
Type string printing special-cased printing of unsafe.Pointer because
it's a built-in type; yet it's declared in a package like any other
imported or used-defined type (unlike built-in types such as int).
Use the same mechanism for printing unsafe.Pointer like any other
(non-basic) type. This will make it possible to use the package
Qualifier if so desired.
Fixes #44515.
Change-Id: I0dd1026f850737ecfc4bb99135cfb8e3c18be9e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/295271
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>