to low-level sync operations.
The only one present at the moment is MemoryFence(), and only for the platforms
for which I could easily discern the proper way to do it. If your favorite platform
isn't represented, patches are welcome!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71770 91177308-0d34-0410-b5e6-96231b3b80d8