test: add bug469, a case where gccgo gaves an incorrect error

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6856084

test/fixedbugs/bug469.go

+// compile
+
+// 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.
+
+// The gccgo compiler would complain about a redefinition of i, but
+// the spec imposes no requirements on parameter names in a function
+// type.
+
+package p
+
+type F func(i int) (i int)