cmd/compile/internal/types2: instantiate methods when instantiating Named types

cmd/compile/internal/types2: instantiate methods when instantiating Named types

This is a port of CL 349412 from go/types to types2 with
minor adjustments for types2 names, plus CL 350143 (slightly
simplified) to make sure we always get a new signature in
instantiated methods, plus CL 350810 to take care of pointer
receivers. It also contains adjustments to the compiler (provided
by Dan Scales) make it work with the types2 changes.

Change-Id: Ia683a3a8adba3c369701c411d786092f02e77efe
Reviewed-on: https://go-review.googlesource.com/c/go/+/349998
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>