Undef MemoryFence when compiling on MSVC.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72732 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/System/Atomic.cpp b/lib/System/Atomic.cpp
index 8c61ff1..cefd0bb 100644
--- a/lib/System/Atomic.cpp
+++ b/lib/System/Atomic.cpp
@@ -18,6 +18,7 @@
 
 #if defined(_MSC_VER)
 #include <windows.h>
+#undef MemoryFence
 #endif
 
 void sys::MemoryFence() {