Allow `cl-labels` to use the same (FUNC EXP) bindings as were already added
to `cl-flet` in Emacs-25. The Info doc (mistakenly) already documented this
new feature.
* lisp/emacs-lisp/cl-macs.el (cl--self-tco-on-form): New function.
(cl-labels): Use it to add support for (FUNC EXP) bindings.
* test/lisp/emacs-lisp/cl-macs-tests.el (cl-macs--labels): Add test for
tail-recursive (FUNC EXP) bindings.