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/bounds_check_elimination.h b/compiler/optimizing/bounds_check_elimination.h
index 05cb185..9e98ccf 100644
--- a/compiler/optimizing/bounds_check_elimination.h
+++ b/compiler/optimizing/bounds_check_elimination.h
@@ -23,10 +23,13 @@
 
 class BoundsCheckElimination : public HOptimization {
  public:
-  explicit BoundsCheckElimination(HGraph* graph) : HOptimization(graph, true, "BCE") {}
+  explicit BoundsCheckElimination(HGraph* graph)
+      : HOptimization(graph, true, kBoundsCheckEliminiationPassName) {}
 
   void Run() OVERRIDE;
 
+  static constexpr const char* kBoundsCheckEliminiationPassName = "BCE";
+
  private:
   DISALLOW_COPY_AND_ASSIGN(BoundsCheckElimination);
 };