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>

misc/cgo/errors/issue18452.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 18452: show pos info in undefined name errors
+
+package p
+
+import (
+	"C"
+	"fmt"
+)
+
+func a() {
+	fmt.Println("Hello, world!")
+	C.function_that_does_not_exist() // line 16
+	C.pi                             // line 17
+}

misc/cgo/errors/test.bash

@@ -48,6 +48,7 @@ check issue13830.go
 check issue16116.go
 check issue16591.go
 check issue18889.go
+expect issue18452.go issue18452.go:16 issue18452.go:17
 
 if ! go build issue14669.go; then
 	exit 1

src/cmd/cgo/gcc.go

@@ -421,7 +421,14 @@ func (p *Package) guessKinds(f *File) []*Name {
 	for i, n := range names {
 		switch sniff[i] &^ notSignedIntConst {
 		default:
-			error_(token.NoPos, "could not determine kind of name for C.%s", fixGo(n.Go))
+			var tpos token.Pos
+			for _, ref := range f.Ref {
+				if ref.Name == n {
+					tpos = ref.Pos()
+					break
+				}
+			}
+			error_(tpos, "could not determine kind of name for C.%s", fixGo(n.Go))
 		case notStrLiteral | notType:
 			if sniff[i]&notSignedIntConst != 0 {
 				n.Kind = "uconst"