cmd/compile/internal/types2: add Named.SetTParams and Named.Orig methods

Named.SetTParams sets the type parameters for a named type.

Named.Orig returns the original generic type an instantiated
type is derived from. Added a new field orig for that purpose
and renamed the already existing orig field to fromRHS.

Finally, updated various comments.

Change-Id: Ic9d173e42740422d195713d8bdc62a54dc8c5f54
Reviewed-on: https://go-review.googlesource.com/c/go/+/309832


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