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

[Glenn Morris <rgm@gnu.org>]

Package: emacs
Version: 25.0.50

Compiling foo.el with contents:
    
    (require 'cl-lib)
    
    (defun foo-1 ()
      (foo-2))
    
    (cl-defmethod foo-2 ()
      t)

results in:

   In end of data:
   foo.el:8:1:Warning: the function `foo-2' is not known to be defined.


There is no such warning when using defun instead of cl-defmethod,
or if foo-2 is moved before foo-1.


I see that cl-defmethod macroexpands to something using declare-function.
Adding

  (setq byte-compile-unresolved-functions
       (delq (assq fn byte-compile-unresolved-functions)
             byte-compile-unresolved-functions))

to byte-compile-macroexpand-declare-function solves the issue,
but I'm not sure it is right, since it might stop
byte-compile-arglist-warn doing its job (?).

On the other hand, it would be consistent with what
byte-compile-file-form-autoload does (see comment in the code).

On the other other hand, declare-function's currently have to come
before any use of the function thay are declaring, which is somewhat
intentional.