[dev.typeparams] go/types: add Named.SetTParams and Named.Orig methods
This is a port of CL 309832 to go/types, adjusted to not export the new
API and to amend TestSizeof.
Change-Id: I67efd3ba9b921c8431528eba1cd88ec1f41898bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/324755
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>