Planning to remove __builtin_llvm_memory_barrier

Hi all,

I am planning to remove __builtin_llvm_memory_barrier soon; the
underlying LLVM intrinsic will be going away as part of the LLVM
atomics rewrite. I plan to add an intrinsic __atomic_thread_fence,
with essentially the same semantics as std::atomic_thread_fence, which
should be sufficient for most uses __sync_synchronize does not cover.
Please get in touch with me if you have some other use for