Add byte-compiler warning about useless trailing cond clauses

Add byte-compiler warning about useless trailing cond clauses

Warn about clauses after the default clause, as in

    (cond ((= x 0) (say "none"))
          (t (say "some"))
          (say "goodbye"))

because they are very much an indicator of a mistake (such as
misplaced brackets), and since they are deleted by the optimiser, any
other warnings there are lost and the user wouldn't know that
something is wrong otherwise.

* lisp/emacs-lisp/macroexp.el (macroexp--expand-all): Add warning.
* etc/NEWS: Announce.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-test--with-suppressed-warnings): Add test case.