test: add bug437, a test that used to fail with gccgo at link time

Updates #3391.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6177045

test/fixedbugs/bug437.dir/one.go

+// Copyright 2012 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.
+
+package one
+
+type I1 interface {
+	f()
+}
+
+type S1 struct {
+}
+
+func (s S1) f() {
+}
+
+func F1(i1 I1) {
+}

test/fixedbugs/bug437.dir/two.go

+// Copyright 2012 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.
+
+package two
+
+import "./one"
+
+type S2 struct {
+	one.S1
+}

test/fixedbugs/bug437.go

+// $G $D/$F.dir/one.go && $G $D/$F.dir/two.go && $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2012 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.
+
+// Test converting a type defined in a different package to an
+// interface defined in a third package, where the interface has a
+// hidden method.  This used to cause a link error with gccgo.
+
+package main
+
+import (
+	"./one"
+	"./two"
+)
+
+func F(i1 one.I1) {
+	switch v := i1.(type) {
+	case two.S2:
+		one.F1(v)
+	}
+}
+
+func main() {
+	F(nil)
+}