Fix another ordering constraint with windows.h and comment about
a revers constraint that we got right (by chance).

llvm-svn: 304792
diff --git a/llvm/lib/Support/Atomic.cpp b/llvm/lib/Support/Atomic.cpp
index 80550e2..55910c4 100644
--- a/llvm/lib/Support/Atomic.cpp
+++ b/llvm/lib/Support/Atomic.cpp
@@ -18,6 +18,8 @@
 
 #if defined(_MSC_VER)
 #include <Intrin.h>
+
+// We must include windows.h after Intrin.h.
 #include <windows.h>
 #undef MemoryFence
 #endif