test: add a test for gccgo compiler bug of missing type descriptor

The gccgo compiler did not generate type descriptor for a pointer
to a type alias defined in another package, causing linking error.
The fix is CL 210787. This CL adds a test.

Updates #36085.

Change-Id: I3237c7fedb4d92fb2dc610ee2b88087f96dc2a1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/210858
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue36085.dir/a.go

+package a
+
+type W = map[int32]interface{}

test/fixedbugs/issue36085.dir/b.go

+package main
+
+import "a"
+
+var w a.W
+var X interface{} = &w
+
+func main() {}

test/fixedbugs/issue36085.go

+// compiledir
+
+// Copyright 2019 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 36085: gccgo compiler did not generate type descriptor
+// for pointer to type alias defined in another package, causing
+// linking error.
+
+package ignored