[C++11] Replace LLVM atomics with std::atomic.

With C++11 we finally have a standardized way to specify atomic operations. Use
them to replace the existing custom implemention. Sadly the translation is not
entirely trivial as std::atomic allows more fine-grained control over the
atomicity. I tried to preserve the old semantics as well as possible.

Differential Revision: http://llvm-reviews.chandlerc.com/D2915

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202730 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Statistic.cpp

@@ -76,7 +76,7 @@ void Statistic::RegisterStatistic() {
       StatInfo->addStatistic(this);
 
     TsanHappensBefore(this);
-    sys::MemoryFence();
+    std::atomic_thread_fence(std::memory_order_seq_cst);
     // Remember we have been registered.
     TsanIgnoreWritesBegin();
     Initialized = true;