[libc++] fix condition_variable_any hangs on stop_request (#77127)

When I implemented `condition_variable_any::wait`, I missed the most
important paragraph in the spec:

> The following wait functions will be notified when there is a stop
request on the passed stop_token.
> In that case the functions return immediately, returning false if the
predicate evaluates to false.

From
https://eel.is/c++draft/thread.condition#thread.condvarany.intwait-1.

Fixes #76807