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

[Shankar Kalpathi Easwaran]

  There are things that I consider why lld needs to have its own way of dispatching threads

  a) only few platforms, implement thread pools behind the scenes for std::async
  b) std::async doesnot try to exploit platform specific features / doesnot allow certain features to be overridden (for example : use gcd on darwin, use ConcRT on MSVC).
  c) has its own set of bugs(tough to debug), not that I am saying our implementation would be bug free :)
  d) Easier to disable threads with own implementation.

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