mark all virtuals in the clang plugin as override

In order to help catch Clang API changes.

Change-Id: I5738f5dff405f7a2dd377c983fb32c1c7a2ff921

compilerplugins/clang/unusedvariablecheck.hxx

@@ -22,7 +22,7 @@ class UnusedVariableCheck
     {
     public:
         explicit UnusedVariableCheck( CompilerInstance& compiler );
-        virtual void run();
+        virtual void run() override;
         bool VisitVarDecl( const VarDecl* var );
     };