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.cpp

@@ -961,7 +961,6 @@ void Object::cancel_delete() {
 	_predelete_ok = true;
 }
 
-#ifdef DEBUG_ENABLED
 ObjectRC *Object::_use_rc() {
 	// The RC object is lazily created the first time it's requested;
 	// that way, there's no need to allocate and release it at all if this Object
@@ -989,7 +988,6 @@ ObjectRC *Object::_use_rc() {
 		rc = _rc.load(std::memory_order_acquire);
 	}
 }
-#endif
 
 void Object::set_script_and_instance(const RefPtr &p_script, ScriptInstance *p_instance) {
 	//this function is not meant to be used in any of these ways
@@ -1927,9 +1925,7 @@ Object::Object() {
 	_emitting = false;
 	memset(_script_instance_bindings, 0, sizeof(void *) * MAX_SCRIPT_INSTANCE_BINDINGS);
 	script_instance = nullptr;
-#ifdef DEBUG_ENABLED
 	_rc.store(nullptr, std::memory_order_release);
-#endif
 #ifdef TOOLS_ENABLED
 
 	_edited = false;
@@ -1942,14 +1938,12 @@ Object::Object() {
 }
 
 Object::~Object() {
-#ifdef DEBUG_ENABLED
 	ObjectRC *rc = _rc.load(std::memory_order_acquire);
 	if (rc) {
 		if (rc->invalidate()) {
 			memdelete(rc);
 		}
 	}
-#endif
 
 	if (script_instance) {
 		memdelete(script_instance);