how to answer yes or no automatically in emacs

You can advice semantic-symref with something like :

(defadvice semantic-symref (around stfu activate)
      (flet ((yes-or-no-p (&rest args) t)
             (y-or-n-p (&rest args) t))
        ad-do-it))

Beware that you're locally bypassing all confirmations, so you may catch further (other) questions triggered by semantic-symref itself.

The answer by @huitseeker is quite neat and effective. After four years, with flet and defadvice being obsolete, I wrote the following functions to answer yes automatically. Maybe it's useful for someone.

(defun my/return-t (orig-fun &rest args)
  t)
(defun my/disable-yornp (orig-fun &rest args)
  (advice-add 'yes-or-no-p :around #'my/return-t)
  (advice-add 'y-or-n-p :around #'my/return-t)
  (let ((res (apply orig-fun args)))
    (advice-remove 'yes-or-no-p #'my/return-t)
    (advice-remove 'y-or-n-p #'my/return-t)
    res))

(advice-add 'projectile-kill-buffers :around #'my/disable-yornp)