Fabric: Using #ifndef NDEBUG instead of #ifdef DEBUG

Summary:
Trivial.
Apparently, `DEBUG` is non-standard feature and using `assert` with `DEBUG` is practically asking for bugs. So, if your `assert` relies on some variable which is only defined when `DEBUG` is set, it's easy to get invalid code because NDEBUG and DEBUG can be unsync.
So, we have to use clunky double negative `#ifndef NDEBUG` everywhere where we used DEBUG.

Reviewed By: JoshuaGross

Differential Revision: D15031328

fbshipit-source-id: 036f573e68925741ca46384261885766c87db1e3

ReactCommon/fabric/mounting/ShadowTree.h

@@ -5,7 +5,7 @@
 
 #pragma once
 
-#ifdef DEBUG
+#ifndef NDEBUG
 #define RN_SHADOW_TREE_INTROSPECTION
 #endif