swift234

[driver] Rework the Unix implementation of TaskQueue::execute() to support reading from pipes as they fill, instead of all at once at the end.

Instead of always calling waitpid() and looking for the termination of a task, call poll() and watch for events on the pipes which are used for buffering subtask output. When we receive a POLLIN or POLLPRI event, ask the corresponding Task to read from the pipe; when we receive a POLLHUP or POLLERR event, wait for the subtask to finish and then ask the Task to perform its post-execution tasks.

Now that the Unix implementation properly handles large amounts of subtask output, re-enable the Unix implementation (if supported).

This fixes <rdar://problem/15795234>.

Swift SVN r12293