go/types, types2: fix scoping for iteration variables declared by range clause

Also correct scope position for such variables.
Adjusted some comments.

Fixes #51437.

Change-Id: Ic49a1459469c8b2c7bc24fe546795f7d56c67cb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/389594


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>