### Godot version

4.1.beta3

### System information

Fedora Workstation 38 - Godot 4.1.beta3 - Vulkan (Forward+)

### Issue description

**This is an issue created by the production team, we're working on it**

Currently, it's impossible to call virtual functions within GDScript.

This currently returns an error:
```gdscript
# a.gd
extends Node

func _process(delta: float) -> void:
	pass
```

```gdscript
# main.gd
extends Node

var my_node: Node = preload("res://a.gd").new()

func _process(delta: float) -> void:
	my_node._process(delta)  # Cannot call virtual function "_process()" because it hasn't been defined.
```

### Steps to reproduce

1. Create a script (`a.gd`) based on `Node` that declares `_process()`;
2. Create a main node;
3. Attach a script to the main node;
4. Initialize a variable with a new `a.gd` instance;
5. Try to run `_process()` on that node;
6. Find out that it returns an error.

### Minimal reproduction project

[virtual_functions.zip (updated)](https://github.com/godotengine/godot/files/11823992/virtual_functions.zip)


GDScriptParser::DataType result;
	result.type_source = GDScriptParser::DataType::ANNOTATED_EXPLICIT;
	result.kind = GDScriptParser::DataType::BUILTIN;
	result.builtin_type = Variant::BOOL;
	p_type_test->set_datatype(result);
	if (!p_type_test->operand || !p_type_test->test_type) {
		return;
	}
	reduce_expression(p_type_test->operand);
	GDScriptParser::DataType operand_type = p_type_test->operand->get_datatype();
	GDScriptParser::DataType test_type = type_from_metatype(resolve_datatype(p_type_test->test_type));
	p_type_test->test_datatype = test_type;
	if (!operand_type.is_set() || !test_type.is_set()) {
		return;
	}
	if (p_type_test->operand->is_constant) {
		p_type_test->is_constant = true;
		p_type_test->reduced_value = false;
		if (!is_type_compatible(test_type, operand_type)) {
			push_error(vformat(R"(Expression is of type "%s" so it can't be of type "%s".)", operand_type.to_string(), test_type.to_string()), p_type_test->operand);
		} else if (is_type_compatible(test_type, type_from_variant(p_type_test->operand->reduced_value, p_type_test->operand))) {
			p_type_test->reduced_value = test_type.builtin_type != Variant::OBJECT || !p_type_test->operand->reduced_value.is_null();
		}
		return;
	}
	if (!is_type_compatible(test_type, operand_type) && !is_type_compatible(operand_type, test_type)) {
		if (operand_type.is_hard_type()) {
			push_error(vformat(R"(Expression is of type "%s" so it can't be of type "%s".)", operand_type.to_string(), test_type.to_string()), p_type_test->operand);
		} else {
			downgrade_node_type_source(p_type_test->operand);