* lisp/emacs-lisp/bytecomp.el: Allow a nil destination file

* lisp/emacs-lisp/bytecomp.el: Allow a nil destination file

(byte-compile--default-dest-file): New function, extracted from
byte-compile-dest-file.
(byte-compile-dest-file): Use it.
(byte-compile-dest-file-function): Give it a non-nil default value.
(byte-recompile-file, byte-compile-file): Handle a nil return value
from `byte-compile-dest-file`.

* lisp/progmodes/elisp-mode.el (elisp-flymake--batch-compile-for-flymake):
Tell the compiler not to write the result, instead of writing it to
a dummy temp file.