test: add test case for incorrect gccgo compilation error

Updates #23489

Change-Id: Ie846ccfe4c4d9295857f5da6863ac8f2ac0f2f6a
Reviewed-on: https://go-review.googlesource.com/89935
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue23489.go

+// run
+
+// 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.
+
+// Caused gccgo to issue a spurious compilation error.
+
+package main
+
+type T struct{}
+
+func (*T) Foo() {}
+
+type P = *T
+
+func main() {
+	var p P
+	p.Foo()
+}