cmd/compile/internal/types2: externalize union type sets

This is a port of CL 371756 from go/types to types2 with
minor adjustments due to different error handling or AST.

Updates #50093

Change-Id: Iab6a4634f8fc917bf99df439d31098624085f52a
Reviewed-on: https://go-review.googlesource.com/c/go/+/372474


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