test: add type alias test that caused gccgo to crash

Change-Id: I3b388e4ac05ace5b7768ade03df2bee5bcc26ba8
Reviewed-on: https://go-review.googlesource.com/73790
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

test/fixedbugs/bug505.go

+// compile
+
+// 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 crashed compiling this file with a failed conversion to the
+// alias type when constructing the composite literal.
+
+package p
+
+type I interface{ M() }
+type A = I
+type S struct {
+	f A
+}
+
+func F(i I) S {
+	return S{f: i}
+}