test: add test case where gccgo incorrectly rejected aliases

Updates #23912

Change-Id: I50d06506a8ac91ed99a761a9ff3fd0b03d4c8121
Reviewed-on: https://go-review.googlesource.com/94995
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue23912.go

+// compile
+
+// Copyright 2018 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.
+
+// A couple of aliases cases that gccgo incorrectly gave errors for.
+
+package p
+
+func F1() {
+	type E = struct{}
+	type X struct{}
+	var x X
+	var y E = x
+	_ = y
+}
+
+func F2() {
+	type E = struct{}
+	type S []E
+	type T []struct{}
+	type X struct{}
+	var x X
+	s := S{E{}}
+	t := T{struct{}{}}
+	_ = append(s, x)
+	_ = append(s, t[0])
+	_ = append(s, t...)
+}