go/types: range clause to accept type sets with single underlying types

go/types: range clause to accept type sets with single underlying types

This is a port of CL 357778 to go/types, adjusted to include error codes
and to use the different range statement syntax in go/ast.

Change-Id: Id537c195cd33a8b422a366269ca8730c2a5bccf1
Reviewed-on: https://go-review.googlesource.com/c/go/+/359875
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>