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