go/gcimporter15: backport interface embedding support

Backports golang.org/cl/38392 from go/internal/gcimporter.

Updates golang/go#16369.

Change-Id: Ic805f96e6565590987a5dae9f0f76c206fceab05
Reviewed-on: https://go-review.googlesource.com/38429
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
2 files changed