go/types: fix internal comments and add additional test case

https://go-review.googlesource.com/c/go/+/132355 addressed
a crash and inadvertently fixed #27346; however the comment
added to the type-checker was incorrect and misleading.

This CL fixes the comment, and adds a test case for #27346.

Fixes #27346.
Updates #22467.

Change-Id: Ib6d5caedf302fd42929c4dacc55e973c1aebfe85
Reviewed-on: https://go-review.googlesource.com/133415


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>