WinSDK: improve module map further

Split out the User32 interfaces from the previously owning module
(WinSock2).  This improves the debugging experience and more accurately
reflects the module structure but should not impact the ability to build
the swift runtime.

Author: compnerd

stdlib/public/Platform/winsdk.modulemap

@@ -112,6 +112,16 @@ module WinSDK [system] [extern_c] {
     export *
   }
 
+  module ShellAPI {
+    header "Shlwapi.h"
+    export *
+  }
+
+  module User {
+    header "WinUser.h"
+    export *
+  }
+
   module WinCrypt {
     header "wincrypt.h"
     export *