go/types: determine hasPtrRecv property from source rather than type

LookupFieldOrMethod needs to know if a method receiver is a pointer
type. Until now this was computed from the the method signature's
receiver, which required the method signature to be type-checked.
Furthermore, it required the receiver to be set before the method
signature was fully type-checked in some cases (see issue #6638).

This CL remembers this property during object resolution, when we
know it from the source.

With this CL, method signatures don't need to be type-checked before
they can be looked up; this is a first step towards separating
type checking of types and type-checking of associated methods.

Updates #23203.
Updates #26854.

Change-Id: Ie3eb7976e8fe8176ea1b284fa7471a4b7000f80b
Reviewed-on: https://go-review.googlesource.com/c/139422


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>