swift234

[driver] Disable the Unix implementation of TaskQueue.

The current implementation does not read() from each Task's pipe until execution
finishes. This causes subtasks to block in write() if the pipe is full, which
only happens if there is a large amount of output (such as in -dump-ast mode).

Fixing output buffering and reenabling the Unix TaskQueue implementation is
tracked by <rdar://problem/15795234>.

Swift SVN r12135