Proposal: spawning tasks in parallel

@yuehuang010 (Felix Huang) from the C++ team is interested in a way to invoke tools (tasks) in parallel.

The problem is that tools such as CLang and GCC often only accept a single file at a time and invoking them serially is slow and wastes time. Instead one could parallelize tool invocations in a sort of a MultiToolTask where inner tasks are executed in parallel and not serially.

In the TPL world, an analog would be Task.WhenAll() or Task.WaitAll().