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.)

modules/gdscript/gdscript_function.h

@@ -81,7 +81,7 @@ struct GDScriptDataType {
 				}
 
 				Object *obj = p_variant.operator Object *();
-				if (!obj || !ObjectDB::instance_validate(obj)) {
+				if (!obj) {
 					return false;
 				}
 
@@ -104,7 +104,7 @@ struct GDScriptDataType {
 				}
 
 				Object *obj = p_variant.operator Object *();
-				if (!obj || !ObjectDB::instance_validate(obj)) {
+				if (!obj) {
 					return false;
 				}