Bug 1022420 - Disable NS_NO_VTABLE on clang-cl in order to avoid the compiler warning; r=froydnj

xpcom/base/nscore.h

@@ -256,7 +256,7 @@
 #ifdef NS_NO_VTABLE
 #undef NS_NO_VTABLE
 #endif
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && !defined(__clang__)
 #define NS_NO_VTABLE __declspec(novtable)
 #else
 #define NS_NO_VTABLE