[PATCH] Use std::async instead of our own parallel execution instrumentation.

[Shankar Kalpathi Easwaran]

  I think the main reason behind TaskGroup was to override concurrent libraries that are platform specific available at linktime.

  For example: on windows you would use ConcRT and on Darwin there is a different concurrent library (forgot, which was discussed earlier).

  std::async doesnot allow to use frameworks that best work on different flavors in my opinion for threads.

http://llvm-reviews.chandlerc.com/D1904