go/types, types2: factor out position comparison, share more code

go/types, types2: factor out position comparison, share more code

This CL introduces the new files util.go and util_test.go for both
type checkers; these files factor out functionality that is different
between the type checkers so that more code (that is otherwise mostly
the same) can be generated.

With cmpPos/CmpPos factored out, go/types/scope.go can now be generated.

Change-Id: I35f67e53d83b3c5086a559b1e826db83d38ee217
Reviewed-on: https://go-review.googlesource.com/c/go/+/461596
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>