Merging r278454:

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r278454 | cbieneman | 2016-08-11 18:29:26 -0700 (Thu, 11 Aug 2016) | 3 lines

[CMake] If the compiler supports _Atomic include atomic.c in builtins libraries

This fixes a long-standing TODO by implementing a compiler check for supporting the _Atomic keyword. If the _Atomic keyword is supported by the compiler we should include it in the builtin library sources.
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llvm-svn: 278679