Merging r277300:

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r277300 | dim | 2016-07-31 22:16:59 +0200 (Sun, 31 Jul 2016) | 5 lines

Fix ASan alloca_constant_size.cc test on FreeBSD.

On FreeBSD <alloca.h> does not exist: alloca(3) is defined in <stdlib.h>
instead.

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llvm-svn: 277424