cmd/compile/internal/types2: don't export TypeSet

For now don't export TypeSet in the interest of
keeping the types2 API surface small(er).

This is a clean port of CL 341289 from go/types.

Change-Id: I50c747629f25472f2ec5ba59d7f543ee3c1c423b
Reviewed-on: https://go-review.googlesource.com/c/go/+/344610


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dan Scales <danscales@google.com>