go/gcimporter15: backport double-export fix for aliased objects

Backport of https://golang.org/cl/32575.

Change-Id: Ic4b0794ee440b7ac6275f0ef7dacda20de4fdad0
Reviewed-on: https://go-review.googlesource.com/32577
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
3 files changed