go/types: use nopos instead of token.NoPos to match types2

go/types: use nopos instead of token.NoPos to match types2

This will simplify the generation of go/types files from types2 files.

Change-Id: Ie9c8061346cff098cb884908c7eb569267886594
Reviewed-on: https://go-review.googlesource.com/c/go/+/461082
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>