misc/cgo/test: add test for passing compatible typedefs

Fixes #23720

Change-Id: I28e0c16503bc043f793e0dab19668f7a66313312
Reviewed-on: https://go-review.googlesource.com/118737
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/issue23720.go

+// 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.
+
+// Test that we can pass compatible typedefs.
+// No runtime test; just make sure it compiles.
+
+package cgotest
+
+/*
+typedef int *issue23720A;
+
+typedef const int *issue23720B;
+
+void issue23720F(issue23720B a) {}
+*/
+import "C"
+
+func Issue23720F() {
+	var x C.issue23720A
+	C.issue23720F(x)
+}