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/register_allocator.h b/compiler/optimizing/register_allocator.h
index ff2f106..579f069 100644
--- a/compiler/optimizing/register_allocator.h
+++ b/compiler/optimizing/register_allocator.h
@@ -81,6 +81,8 @@
         + double_spill_slots_.Size();
   }
 
+  static constexpr const char* kRegisterAllocatorPassName = "register";
+
  private:
   // Main methods of the allocator.
   void LinearScan();