Change defgeneric so it doesn't completely redefine the function

Change defgeneric so it doesn't completely redefine the function

* lisp/emacs-lisp/cl-generic.el (cl-generic-define): Don't throw away
previously defined methods.
(cl-generic-define-method): Let-bind purify-flag instead of using `fset'.
(cl--generic-prefill-dispatchers): Only define during compilation.
(cl-method-qualifiers): Remove redundant alias.
(help-fns-short-filename): Silence byte-compiler.
* test/automated/cl-generic-tests.el: Adjust to new defgeneric semantics.