Improve counsel-async initial anchoring

Improve counsel-async initial anchoring

* counsel.el (counsel--async-sentinel): Rely on
  `ivy-recompute-index-swiper-async' even when `ivy--old-cands' is nil.

* ivy.el (ivy--recompute-index): Check for `ivy--old-cands' being nil
  before trying to index it.
(ivy-recompute-index-swiper-async): When `ivy--old-cands' is nil, look
for a string that's closest to the line number at point.