cider-defun-at-point doesn't work correctly with #_ prefixed forms

Partially related to #2375.

`cider-defun-at-point` goes completely nuts when invoked from forms prefixed by `#_` (or any other data reader for that matter). Here's an example (█ stands for cursor):

```clj
#_(foo█)

#_(bar)
```

`(cider-defun-at-point)` here returns `"#_"`! Basically, it returns any form that follows the #_-prefixed form we are in; and that *might* be the desired behavior – but certainly not returning `#_`.