Previously, we used to associate a function with its first returned type
assuming that it is a factory function for that type.

However, a function may return multiple types in which case it is usually
doing something else. Check for multiple return types, and treat it as
a normal function in that case. Maintain same behavior if the function
returns just one type.

Fixes #12839

Change-Id: Ic4ac11d322996f216f593b71f4e61ad4270d5213
Reviewed-on: https://go-review.googlesource.com/105575Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>