ART: Add -Wunused

Until the global CFLAGS are fixed, add Wunused. Fix declarations
in the optimizing compiler.

Change-Id: Ic4553f08e809dc54f3d82af57ac592622c98e000
diff --git a/compiler/optimizing/gvn.h b/compiler/optimizing/gvn.h
index 57e0487..e74d969 100644
--- a/compiler/optimizing/gvn.h
+++ b/compiler/optimizing/gvn.h
@@ -27,10 +27,12 @@
 class GVNOptimization : public HOptimization {
  public:
   GVNOptimization(HGraph* graph, const SideEffectsAnalysis& side_effects)
-      : HOptimization(graph, true, "GVN"), side_effects_(side_effects) {}
+      : HOptimization(graph, true, kGlobalValueNumberingPassName), side_effects_(side_effects) {}
 
   void Run() OVERRIDE;
 
+  static constexpr const char* kGlobalValueNumberingPassName = "GVN";
+
  private:
   const SideEffectsAnalysis& side_effects_;