Bug 1347040 - Prevent -Woverloaded-virtual of AccessibleWrap.h when __clang__ defined r=Jamie

Some classes of a11y are defined from both IUnknown and nsISupports.  Both of
them have QueryInterface().  So, one of them is overridden by a sub class
hides the other and this is warned by clang.   However, AccessibleWrap.h
prevents this warning since it's by design when the compiler is GCC.
So, it should do same thing when the compiler is clang.

Differential Revision: https://phabricator.services.mozilla.com/D4310

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

accessible/windows/msaa/AccessibleWrap.h

@@ -20,7 +20,7 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/mscom/Utils.h"
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 // Inheriting from both XPCOM and MSCOM interfaces causes a lot of warnings
 // about virtual functions being hidden by each other. This is done by
 // design, so silence the warning.