cmd/cgo: annotate named return struct members in comments

cmd/cgo: annotate named return struct members in comments

If an exported function has named return variables, then show the names
as comments in the return struct we create in the header file.

Example here:

 https://groups.google.com/forum/#!topic/golang-nuts/r393ne4zIfY

Change-Id: I21fb4ca2673f6977bec35ccab0cef7d42b311f96
Reviewed-on: https://go-review.googlesource.com/13061
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>