The standard go/internal/gccgoimporter package is essentially unusable
behind the go/importer.For API, so this change makes a copy of it in
x/tools. A follow-up change will expose a small but usable API to it.
Change-Id: Ica5092267ecafb78e1d983c86aa46e4e0bef02d5
Reviewed-on: https://go-review.googlesource.com/31854
Reviewed-by: Robert Griesemer <gri@golang.org>
Alan Donovan