Merging r367084:

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r367084 | kongyi | 2019-07-26 07:17:14 +0200 (Fri, 26 Jul 2019) | 6 lines

Fix macOS build after r358716

COPYFILE_CLONE is only defined on newer macOS versions, using it without
check breaks build on systems running legacy OS and toolchain.

Differential Revision: https://reviews.llvm.org/D65317
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llvm-svn: 368848