cmd/compile/internal/types2: use the orig object for Named.Obj

Exposing a synthetic type name for instantiated types is problematic:
there is no way to ensure that type instances are first created in the
same type checking pass, and therefore no guarantee that their
instantiation positions are the same. Even type checking a given package
with different file ordering could result in different positions being
associated with type instances. This is therefore an implementation
detail that we should not expose.

Keep the synthetic type name for accurate error reporting, but hide it
in the API.

Change-Id: I61f0e3ed322e97b157eb1ca316480f5719dcc174
Reviewed-on: https://go-review.googlesource.com/c/go/+/341858


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>