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_builder.h b/compiler/optimizing/ssa_builder.h
index 148e959..f50da46 100644
--- a/compiler/optimizing/ssa_builder.h
+++ b/compiler/optimizing/ssa_builder.h
@@ -60,6 +60,8 @@
 
   static HInstruction* GetReferenceTypeEquivalent(HInstruction* instruction);
 
+  static constexpr const char* kSsaBuilderPassName = "ssa_builder";
+
  private:
   // Locals for the current block being visited.
   HEnvironment* current_locals_;