go/types, types2: add testcase for alias issue

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>