[dev.typeparams] cmd/compile/internal/types: format union types

Previously it was just printing <S>. Now it prints things like int32|~int64.

Change-Id: I960b011ce8ed360020a49ae7809d85d1d1fdbfb2
Reviewed-on: https://go-review.googlesource.com/c/go/+/336692


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>