Preserve setf semantics in 'substring', 'cons', 'logand' expanders

* doc/lispref/variables.texi (Adding Generalized Variables): Fix example.
* lisp/emacs-lisp/cl-lib.el (substring)
* lisp/emacs-lisp/gv.el (cons, logand): Return the value being
assigned, as specified for 'setf'.  (bug#35546)

lisp/emacs-lisp/cl-lib.el

@@ -619,8 +619,11 @@ If ALIST is non-nil, the new pairs are prepended to it."
       (macroexp-let2* nil ((start from) (end to))
         (funcall do `(substring ,getter ,start ,end)
                  (lambda (v)
-                   (funcall setter `(cl--set-substring
-                                     ,getter ,start ,end ,v))))))))
+                   (macroexp-let2 nil v v
+                     `(progn
+                        ,(funcall setter `(cl--set-substring
+                                           ,getter ,start ,end ,v))
+                        ,v))))))))
 
 ;;; Miscellaneous.