[release-branch.go1.18] go/types, types2: use correct underlying type in union set...

[release-branch.go1.18] go/types, types2: use correct underlying type in union set computation

Fixes #51665.

Change-Id: Ibf415d7e12849b8f50b58d74713613d4e65bc347
Reviewed-on: https://go-review.googlesource.com/c/go/+/392575
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/392577
TryBot-Result: Gopher Robot <gobot@golang.org>