go/gcimporter: fix importing of anonymous interfaces

This is a backport of https://go-review.googlesource.com/#/c/41198/.

For golang/go#20046.

Change-Id: I58448c7dcc5e835d5c774e253cb56fec6e154f12
Reviewed-on: https://go-review.googlesource.com/41204
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
3 files changed