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/ssa_liveness_analysis.h b/compiler/optimizing/ssa_liveness_analysis.h
index 0e68a61..be72629 100644
--- a/compiler/optimizing/ssa_liveness_analysis.h
+++ b/compiler/optimizing/ssa_liveness_analysis.h
@@ -816,6 +816,8 @@
     return number_of_ssa_values_;
   }
 
+  static constexpr const char* kLivenessPassName = "liveness";
+
  private:
   // Linearize the graph so that:
   // (1): a block is always after its dominator,