go/types, types2: add testcase for alias issue

Using the new type Alias node, this issue is now fixed.
Add a test case.

Fixes #50729.

Change-Id: I22a4cf31b83de497e052989ca2054227e65e9937
Reviewed-on: https://go-review.googlesource.com/c/go/+/546455
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: griesemer

src/internal/types/testdata/fixedbugs/issue50729b.go

@@ -0,0 +1,15 @@
+// -gotypesalias=1
+
+// Copyright 2023 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.
+
+package p
+
+type d[T any] struct{}
+type (
+	b d[a]
+)
+
+type a = func(c)
+type c struct{ a }