Add the function declaration and property `important-return-value`

Add the function declaration and property `important-return-value`

Now the declaration

 (declare (important-return-value t))

can be used to have the byte-compiler warn when the return value from
a call is discarded (bug#61730).

* lisp/emacs-lisp/bytecomp.el (byte-compile-form)
(important-return-value-fns): Use the function property
`important-return-value` instead of looking through a static list.
* lisp/emacs-lisp/byte-run.el (byte-run--set-important-return-value)
(defun-declarations-alist): New function declaration, setting the
property of the same name.
* lisp/emacs-lisp/cl-macs.el:
* lisp/subr.el (assoc-default): Set the property.
* doc/lispref/functions.texi (Declare Form):
* doc/lispref/symbols.texi (Standard Properties): Document.
* etc/NEWS: Announce.