Merging r311269:

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r311269 | jlpeyton | 2017-08-19 16:53:36 -0700 (Sat, 19 Aug 2017) | 8 lines

Use va_copy instead of __va_copy to fix building libomp against musl libc

Fixes https://bugs.llvm.org/show_bug.cgi?id=34040

Patch by Peter Levine

Differential Revision: https://reviews.llvm.org/D36343

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llvm-svn: 318175