go/types: update interface receivers after substituting

Interface method receivers are synthetic: they record either the
interface type or the the defined type for which they are the RHS of the
type declaration. When instantiating, we need to update these receivers
accordingly.

Fixes #50839

Change-Id: Icd8e1a2817b0135059d25d034b01b0ff5207641f
Reviewed-on: https://go-review.googlesource.com/c/go/+/381174


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>