Do call debugger on failed cl-assert

Do call debugger on failed cl-assert

"Don't call debug on failed cl-assert..." removed the call to `debug' in
cl--assertion-failed because `debug' calls `kill-emacs' in batch mode,
thus messing up ert test runs.  However, calling the debugger is useful
because it allows catching failed assertions even inside
`condition-case' calls.  The problem with ert can be avoided by calling
`debugger' instead of `debug' directly, since ert installs its own
debugger while running tests.

* lisp/emacs-lisp/cl-preloaded.el (cl--assertion-failed): Call
`debugger' if `debug-on-error' is non-nil.