[dev.typeparams] cmd/compile/internal/types2: better recv Var for method expressions

When synthesizing the Signature to represent a method expression, keep
as much of the original parameter as possible, substituting only the
receiver type.

Fixes #46209.

Change-Id: Ic4531820ae7d203bb0ba25a985f72d219b4aa25f
Reviewed-on: https://go-review.googlesource.com/c/go/+/320489


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>