[dev.typeparams] cmd/compile: change types2.Union API to accept a list of Terms

Instead of providing a list of tildes and types, use a list of
Terms to create a Union, with suitable accessors.

Define the (exported) notion of a Term representing a union term.

This simplified various uses and also will be easier to extend
should we want to add more information to a Term in the future.

Change-Id: I52fd73938bfa11bac60adbf10580b6d0680df4f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/340250


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>