It's awkward to get a string value in cgoCheckArg, but SWIG testing
revealed that it is possible. The new handling of extra files in the
ptr.go test emulates what SWIG does with an exported function that
returns a string.
Change-Id: I453717f867b8a49499576c28550e7c93053a0cf8
Reviewed-on: https://go-review.googlesource.com/19020
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Russ Cox <rsc@golang.org>