[dev.typeparams] go/types: adjust logic for method expression arg naming

CL 325369 improved this logic in types2. Port this improvement back to
go/types.

Change-Id: I5f859cbffd88bb3db09a81c2389269f7bd0869f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/330069


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>