Pull in r264335 from upstream llvm trunk:

Pull in r264335 from upstream llvm trunk:

  Add <atomic> to ThreadPool.h, since std::atomic is used

  Summary:
  Apparently, when compiling with gcc 5.3.2 for powerpc64, the order of
  headers is such that it gets an error about std::atomic<> use in
  ThreadPool.h, since this header is not included explicitly.  See also:

  https://llvm.org/bugs/show_bug.cgi?id=27058

  Fix this by including <atomic>.  Patch by Bryan Drewery.

  Reviewers: chandlerc, joker.eph

  Subscribers: bdrewery, llvm-commits

  Differential Revision: http://reviews.llvm.org/D18460