Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/compiler/utils/arena_object.h b/compiler/utils/arena_object.h
index 50909f75..8f6965e 100644
--- a/compiler/utils/arena_object.h
+++ b/compiler/utils/arena_object.h
@@ -31,6 +31,7 @@
 
   void operator delete(void*, size_t) {
     LOG(FATAL) << "UNREACHABLE";
+    UNREACHABLE();
   }
 };