### Godot version

v4.2.rc.mono.custom_build [ad72de508]

### System information

Windows 10

### Issue description

As I pointed out several times earlier in [#80284](https://github.com/godotengine/godot/pull/80284#issuecomment-1722120503), the references to singletons in `GDScript` are still not updated after the `GDExtension` reload.

https://github.com/godotengine/godot/assets/7782218/f90052a5-0de2-4310-8235-f0b9e2f74b3a

Singletons themselves are updated and work correctly, but references to these singletons are not updated in `@tool` scripts. That is, `Engine.get_singleton` returns the actual working reference, but using the singleton directly through its name causes errors.

### Steps to reproduce

* Clone `4.2.reload_singleton` branch from the MRP repository https://github.com/DmitriySalnikov/gde_test_cases/tree/4.2.reload_singleton
* Build as usual
* Open the project from the `project/` folder
* Add one character to the source code and rebuild
* Focus on the editor to reload extension
* Watch for error spam when calling by singleton name

### Minimal reproduction project

https://github.com/DmitriySalnikov/gde_test_cases/tree/4.2.reload_singleton
int gcc = CoreConstants::get_global_constant_count();
	for (int i = 0; i < gcc; i++) {
		_add_global(StaticCString::create(CoreConstants::get_global_constant_name(i)), CoreConstants::get_global_constant_value(i));
	}
	_add_global(StaticCString::create("PI"), Math_PI);
	_add_global(StaticCString::create("TAU"), Math_TAU);
	_add_global(StaticCString::create("INF"), INFINITY);
	_add_global(StaticCString::create("NAN"), NAN);
	
	List<StringName> class_list;
	ClassDB::get_class_list(&class_list);
	for (const StringName &n : class_list) {
		if (globals.has(n)) {
			continue;
		}
		Ref<GDScriptNativeClass> nc = memnew(GDScriptNativeClass(n));
		_add_global(n, nc);
	}
	
	List<Engine::Singleton> singletons;
	Engine::get_singleton()->get_singletons(&singletons);
	for (const Engine::Singleton &E : singletons) {
		_add_global(E.name, E.ptr);
	}
	GDScript::func_ptrs_to_update_thread_local = &GDScript::func_ptrs_to_update_main_thread;
#ifdef TESTS_ENABLED
	GDScriptTests::GDScriptTestRunner::handle_cmdline();
#endif
}
String GDScriptLanguage::get_type() const {