go/types: delay union element checks

This is a clean port of CL 351969 from types2 to go/types
with a minor adjustment for error handling (provide an error
code).

For #46461.

Change-Id: I493dde12d8ccf86aa33f4913ac6e82f2eb459088
Reviewed-on: https://go-review.googlesource.com/c/go/+/351971


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>