cmd/compile/internal/types2: delay union element checks

We cannot determine the underlying type right when parsing
a union term since it may lead to types that are not yet
fully set up.

Fixes #46461.

Change-Id: I1fcadb1dcef2160be2f088a4a34e99dbab01da67
Reviewed-on: https://go-review.googlesource.com/c/go/+/351969


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>