cmd/cgo: show pos info in undefined name errors

cmd/cgo: show pos info in undefined name errors

For test.go:

	package main

	import (
	   "C"
	   "fmt"
	)

	func main() {
		 fmt.Println("Hello, world!")
		 C.no_such_f()
	}

Before:

	could not determine kind of name for C.no_such_f

After:

	./test.go:10:2: could not determine kind of name for C.no_such_f

Fixes #18452

Change-Id: I49c136b7fa60fab25d2d5b905d440fe4d106e565
Reviewed-on: https://go-review.googlesource.com/34783
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>