Use ATTRIBUTE_UNUSED more.

Use it in lieu of UNUSED(), which had some incorrect uses.

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/base/scoped_arena_allocator.h b/runtime/base/scoped_arena_allocator.h
index 4f51370..2554fb0 100644
--- a/runtime/base/scoped_arena_allocator.h
+++ b/runtime/base/scoped_arena_allocator.h
@@ -132,7 +132,7 @@
   ScopedArenaAllocatorAdapter<void> Adapter(ArenaAllocKind kind = kArenaAllocSTL);
 
   // Allow a delete-expression to destroy but not deallocate allocators created by Create().
-  static void operator delete(void* ptr) { UNUSED(ptr); }
+  static void operator delete(void* ptr ATTRIBUTE_UNUSED) {}
 
  private:
   ArenaStack* const arena_stack_;