Due to the nature of this class, there are two implementations of TaskQueue:
a Unix-specific implementation which supports both parallel execution and output
buffering, and a default implementation which supports neither of these features.
(The default implementation uses the functions from llvm/Support/Program.h for execution.)
TaskQueue allows clients to provide a TaskBeganCallback and a TaskFinishedCallback,
each of which will be called when a new task begins or when a task finishes execution,
respectively. Clients may add tasks to the TaskQueue in either of these callbacks,
and clients can stop further execution by returning TaskFinishedResponse::StopExecution
from the TaskFinishedCallback.
Swift SVN r12059
