cl-generic: Fix `advertised-calling-convention` declarations

cl-generic: Fix `advertised-calling-convention` declarations

* lisp/emacs-lisp/cl-generic.el (cl-generic-define-method):
Preserve the `advertised-calling-convention`, if any (bug#58563).

* lisp/subr.el (declare): Warn when we hit this.

* lisp/emacs-lisp/byte-run.el (get-advertised-calling-convention): New fun.
* lisp/progmodes/elisp-mode.el (elisp-get-fnsym-args-string):
* lisp/help-fns.el (help-fns--signature):
* lisp/emacs-lisp/bytecomp.el (byte-compile-fdefinition): Use it.

* test/lisp/emacs-lisp/cl-generic-tests.el (cl-generic-tests--acc): New fun.
(cl-generic-tests--advertised-calling-convention-bug58563): New test.