Use ATTRIBUTE_UNUSED more.

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

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/base/arena_containers.h b/runtime/base/arena_containers.h
index 9174d2d..e2d4c24 100644
--- a/runtime/base/arena_containers.h
+++ b/runtime/base/arena_containers.h
@@ -176,8 +176,8 @@
   pointer address(reference x) const { return &x; }
   const_pointer address(const_reference x) const { return &x; }
 
-  pointer allocate(size_type n, ArenaAllocatorAdapter<void>::pointer hint = nullptr) {
-    UNUSED(hint);
+  pointer allocate(size_type n,
+                   ArenaAllocatorAdapter<void>::pointer hint ATTRIBUTE_UNUSED = nullptr) {
     DCHECK_LE(n, max_size());
     return arena_allocator_->AllocArray<T>(n, ArenaAllocatorAdapterKind::Kind());
   }