[C++11] Replace LLVM atomics with std::atomic.
With C++11 we finally have a standardized way to specify atomic operations. Use
them to replace the existing custom implemention. Sadly the translation is not
entirely trivial as std::atomic allows more fine-grained control over the
atomicity. I tried to preserve the old semantics as well as possible.
Differential Revision: http://llvm-reviews.chandlerc.com/D2915
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202730
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