Add MSVC support to Singleton's fatalHelper

Summary: MSVC doesn't support constructor priorities, so use the same code as OSX.
Closes #272

Reviewed By: @yfeldblum

Differential Revision: D2283998

Pulled By: @sgolemon

folly/Singleton.cpp

@@ -45,7 +45,7 @@ struct FatalHelper {
   std::vector<detail::TypeDescriptor> leakedSingletons_;
 };
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(_MSC_VER)
 // OS X doesn't support constructor priorities.
 FatalHelper fatalHelper;
 #else