test: add test case that gccgo miscompiled

Error was

main.go:7:11: error: import error at 162: expected ‘<type ’

Change-Id: Iacfe4bfa003d7708a21ebc89ad1ab2d4a3b041a8
Reviewed-on: https://go-review.googlesource.com/70290
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

test/fixedbugs/bug504.dir/a.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.
+
+package a
+
+type MyInt = int

test/fixedbugs/bug504.dir/b.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.
+
+package b
+
+import "./a"
+
+func F() a.MyInt {
+	return 0
+}

test/fixedbugs/bug504.dir/c.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.
+
+package c
+
+import "./b"
+
+var V = b.F()

test/fixedbugs/bug504.dir/main.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.
+
+package main
+
+import "./c"
+
+func main() {
+	println(c.V)
+}

test/fixedbugs/bug504.go

+// compiledir
+
+// 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.
+
+// Gccgo mishandled a reference to a type alias in a package that was
+// not directly imported.
+
+package ignored