Promote object validity checks to release builds

Extra:
- Optimized the debug-only check about why the object is null to determine if it's because it has been deleted (the RC is enough; no need to check the ObjectDB).
- Because of the previous point. the debugger being attached is not required anymore for giving the "Object was deleted" error; from now, it only matters that it's a debug build.
- `is_instance_valid()` is now trustworthy. It will return `true` if, and only if, the last object assigned to a `Variant` is still alive (and not if a new object happened to be created at the same memory address of the old one).
- Replacements of `instance_validate()` are used where possible `Variant::is_invalid_object()` is introduced to help with that. (GDScript's `is_instance_valid()` is good.)

core/object.h

@@ -41,9 +41,7 @@
 #include "core/variant.h"
 #include "core/vmap.h"
 
-#ifdef DEBUG_ENABLED
-#include <atomic> // For ObjectRC*
-#endif
+#include <atomic>
 
 #define VARIANT_ARG_LIST const Variant &p_arg1 = Variant(), const Variant &p_arg2 = Variant(), const Variant &p_arg3 = Variant(), const Variant &p_arg4 = Variant(), const Variant &p_arg5 = Variant()
 #define VARIANT_ARG_PASS p_arg1, p_arg2, p_arg3, p_arg4, p_arg5
@@ -476,9 +474,7 @@ private:
 	int _predelete_ok;
 	Set<Object *> change_receptors;
 	ObjectID _instance_id;
-#ifdef DEBUG_ENABLED
 	std::atomic<ObjectRC *> _rc;
-#endif
 	bool _predelete();
 	void _postinitialize();
 	bool _can_translate;
@@ -590,9 +586,7 @@ public:
 		return &ptr;
 	}
 
-#ifdef DEBUG_ENABLED
 	ObjectRC *_use_rc();
-#endif
 
 	bool _is_gpl_reversed() const { return false; }
 
@@ -798,6 +792,7 @@ public:
 	static void debug_objects(DebugFunc p_func);
 	static int get_object_count();
 
+	// This one may give false positives because a new object may be allocated at the same memory of a previously freed one
 	_FORCE_INLINE_ static bool instance_validate(Object *p_ptr) {
 		rw_lock.read_lock();