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/side_effects_analysis.h b/compiler/optimizing/side_effects_analysis.h
index f1c98ac..415d10c 100644
--- a/compiler/optimizing/side_effects_analysis.h
+++ b/compiler/optimizing/side_effects_analysis.h
@@ -25,7 +25,7 @@
 class SideEffectsAnalysis : public HOptimization {
  public:
   explicit SideEffectsAnalysis(HGraph* graph)
-      : HOptimization(graph, true, "SideEffects"),
+      : HOptimization(graph, true, kSideEffectsAnalysisPassName),
         graph_(graph),
         block_effects_(graph->GetArena(), graph->GetBlocks().Size(), SideEffects::None()),
         loop_effects_(graph->GetArena(), graph->GetBlocks().Size(), SideEffects::None()) {}
@@ -38,6 +38,8 @@
 
   bool HasRun() const { return has_run_; }
 
+  static constexpr const char* kSideEffectsAnalysisPassName = "SideEffects";
+
  private:
   void UpdateLoopEffects(HLoopInformation* info, SideEffects effects);