[WinSDK] Modularize <winapifamily.h>

This prevents other Clang modules from hijacking this header, causing modularization errors. For example, several WinRT headers `#include <winapifamily.h>`.

Author: egorzhdan

stdlib/public/Platform/winsdk_shared.modulemap

@@ -14,3 +14,8 @@ module _GUIDDef {
   header "guiddef.h"
   export *
 }
+
+module WinAPIFamily {
+  header "winapifamily.h"
+  export *
+}