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