Struct field ambiguity test.

gccgo gets this wrong at the moment.

R=rsc
CC=golang-dev
https://golang.org/cl/205044

test/fixedbugs/bug253.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug253
+
+// Copyright 2010 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 main
+
+type S1 struct {
+	i int
+}
+type S2 struct {
+	i int
+}
+type S3 struct {
+	S1
+	S2
+}
+type S4 struct {
+	S3
+	S1
+}
+func main() {
+	var s4 S4
+	if s4.i != 0 {	// .i refers to s4.S1.i, unambiguously
+		panic()
+	}
+}