[dev.typeparams] go/types: better recv Var for method expressions

This is a port of CL 320489 to go/types, adjusted to be consistent about
named/unnamed parameters. TestEvalPos was failing without this addition.

For #46209

Change-Id: Icdf86e84ebce8ccdb7846a63b5605e360e2b8781
Reviewed-on: https://go-review.googlesource.com/c/go/+/324733


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>