bug#31641: 26.1; iter-do variable not left unused warning

[Christopher Wellons]

When byte-compiling an iter-do form with a variable intended to be left 
unused, the compiler emits a false warning:

;;; -*- lexical-binding: t; -*-
(require 'generator)
(iter-do (_ i))
;; -> "Warning: variable ‘_’ not left unused"

Giving the variable a name has the opposite effect, though it's not 
a false warning in this case:

(iter-do (v i))
;; -> "Unused lexical variable ‘v’"

This issue does not occur with similar, long-established forms:

;;; -*- lexical-binding: t; -*-
(dolist (_ '(a b c)))
(dotimes (_ 10))
;; -> no warnings