[dev.typeparams] go/types: move embedding positions from Checker to Interface
This is a straightforward port of CL 331514 to go/types, with minor
adjustments for the different position API.
Change-Id: I714b3f1cd5a0e8d249912bb589d456885a87e167
Reviewed-on: https://go-review.googlesource.com/c/go/+/335030
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>