[WinSDK] Modularize <guiddef.h>

This fixes modularization errors that arise when importing a C++ header that contains `#include <guiddef.h>`, which might hijack this header from the WinSDK module where it belongs.

test/stdlib/WinSDK_GUID.swift

@@ -0,0 +1,9 @@
+// RUN: %target-build-swift %s
+// REQUIRES: OS=windows-msvc
+
+// Make sure that importing WinSDK brings in the GUID type, which is declared in
+// /shared and not in /um.
+
+import WinSDK
+
+public func usesGUID(_ x: GUID) {}