Children of hidden elements not exposed through labelledby

Provided this input:
```
<img src="foo.jpg" id="test" alt="test" aria-labelledby="t1">
<div id="t1" style="visibility:hidden">foo</div>
```
The calculated accname is "foo", but if we add an internal, inline node like this:
```
<img src="foo.jpg" id="test" alt="test" aria-labelledby="t1">
<div id="t1" style="visibility:hidden"><span>foo</span></div>
```
The result is "test".