Fix warning-suppress for list type "warning type"

Fix warning-suppress for list type "warning type"

Per the documentation of 'warning-suppress-types' and the
implementation of 'warning-suppress-p', a warning type can
be either a symbol or a list of symbols.  The previous
implementation could generate wrong 'warning-suppress-types':

  old behavior:
  type              warning-suppress-types
  pkg           ->    '((pkg))                  Correct
  (pkg subtype) ->    '(((pkg subtype)))        Incorrect

Now we check whether type is a cons cell first.  (Should not
use listp here, as listp returns t for nil.)

  new behavior:
  type              warning-suppress-types
  pkg           ->    '((pkg))                Correct
  (pkg subtype) ->    '((pkg subtype))        Correct

* lisp/emacs-lisp/warnings.el (warnings-suppress): Fix saving
warning types in 'warning-suppress-types'.  (Bug#70063)

Copyright-paperwork-exempt: yes