bug#20509: compiler warns about cl-defmethod defined after use

[Stefan Monnier]

>> While using cl-defmethod without a previous cl-defgeneric is supported,
>> it's not recommended.
> It's used without one in several places in Emacs:

That's because EIEIO's older `defmethod' always counted as
a "full" definition.  We can fix those warnings in one of the following ways:
- Add a corresponding cl-defgeneric.
- Add a corresponding `require' which loads the package where the
  corresponding `cl-defgeneric' is found.
- Reorder the code so that the cl-defmethod appears before the call(s).
- Change cl-generic.el to use the same kind of bytecompiler hack that
  eieio used to use instead if using `declare-function'.


        Stefan