test: add test for function type in function literal

The gccgo compiler used to fail this test.  This was the root
cause of http://gcc.gnu.org/PR61308 .  The fix for the gccgo
compiler is https://golang.org/cl/122020043 .

LGTM=dave, bradfitz
R=golang-codereviews, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/121200043

test/fixedbugs/bug489.go

+// compile
+
+// Copyright 2014 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.
+
+// The gccgo compiler had a bug: mentioning a function type in an
+// expression in a function literal messed up the list of variables
+// referenced in enclosing functions.
+
+package main
+
+func main() {
+	v1, v2 := 0, 0
+	f := func() {
+		a := v1
+		g := (func())(nil)
+		b := v2
+		_, _, _ = a, g, b
+	}
+	_, _, _ = v1, v2, f
+}