cmd/compile/internal/types2: export Named.Orig as Named.Origin

This is a slightly adjusted port of CL 350996 from go/types to types2.

Change-Id: I8f5902ecb20a4b4d2a5ef0d3641d857bc3a618e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/351170


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>