go/types: export Named._Orig as Named.Origin

Export the _Orig method for Named types, and rename to Origin.  As
discussed in #47916, Orig is easily confused with Original, which is
not as good a name. Spelling out 'Origin' resolves the ambiguity.

Updates #47916

Change-Id: I377c73de82310d3f6aa0bd82dddcd60ee067155b
Reviewed-on: https://go-review.googlesource.com/c/go/+/350996


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>