test: add test that causes gofrontend crash

For #52846

Change-Id: I763f81def97b53277396c123c524f7b8193ea35e
Reviewed-on: https://go-review.googlesource.com/c/go/+/413694
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

test/fixedbugs/issue52846.go

@@ -0,0 +1,17 @@
+// compile
+
+// Copyright 2022 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.
+
+// Issue 52846: gofrontend crashed with alias as map key type
+
+package p
+
+type S struct {
+	F string
+}
+
+type A = S
+
+var M = map[A]int{A{""}: 0}