Merging r296063:

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r296063 | hans | 2017-02-23 17:16:34 -0800 (Thu, 23 Feb 2017) | 4 lines

Revert r291477 "[Frontend] Correct values of ATOMIC_*_LOCK_FREE to match builtin"

It caused PR31864. There is a patch in progress to fix that, but let's
revert in the meantime.
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llvm-svn: 296152