The function has a bizarre signature: it was the only public function there
that exposed the reflect package. Also, its definition is peculiar and hard to
explain. It doesn't merit being exported.

This is an API change but really, it should never have been exported and
it's certain very few programs will depend on it: it's too weird.

Fixes #2846.

R=golang-dev, gri, bradfitz
CC=golang-dev
https://golang.org/cl/5639054