* lisp/emacs-lisp/eieio-core.el: Add `subclass' specializer for cl-generic.

* lisp/emacs-lisp/eieio-core.el: Add `subclass' specializer for cl-generic.

(eieio--generic-subclass-tagcode, eieio--generic-subclass-tag-types):
New functions.
(cl-generic-tagcode-function, cl-generic-tag-types-function): Use them.

* test/automated/eieio-test-methodinvoke.el (eieio-test-cl-generic-1):
Test `subclass' specializer.