Bug 1521690 - Use Unused << instead of (void*) in nsWindow.cpp r=aklotz,jmathies

Casting non-void result to `void*` causes warning of clang.  Additionally,
perhaps, we should use `Unused <<` because of modern style.

And also this patch makes widget/windows is treated as "warning as errors"
because this patch fixes the last warning.

Differential Revision: https://phabricator.services.mozilla.com/D17216

--HG--
extra : moz-landing-system : lando

widget/windows/moz.build

@@ -160,6 +160,3 @@ if CONFIG['CC_TYPE'] in ('msvc', 'clang-cl'):
     SOURCES['ToastNotification.cpp'].flags += ['-wd5038']
     SOURCES['ToastNotificationHandler.cpp'].flags += ['-wd5038']
     SOURCES['WindowsUIUtils.cpp'].flags += ['-wd5038']
-
-if CONFIG['CC_TYPE'] == 'clang-cl':
-    AllowCompilerWarnings()  # workaround for bug 1090497

widget/windows/nsWindow.cpp

@@ -4928,7 +4928,7 @@ bool nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
 
     case MOZ_WM_STARTA11Y:
 #if defined(ACCESSIBILITY)
-      (void*)GetAccessible();
+      Unused << GetAccessible();
       result = true;
 #else
       result = false;