### 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)


case GDScriptParser::IdentifierNode::MEMBER_CONSTANT:
			p_identifier->set_datatype(p_identifier->constant_source->get_datatype());
			p_identifier->is_constant = true;
			
			p_identifier->reduced_value = p_identifier->constant_source->initializer->reduced_value;
			found_source = true;
			break;
		case GDScriptParser::IdentifierNode::MEMBER_SIGNAL:
		case GDScriptParser::IdentifierNode::INHERITED_VARIABLE:
			mark_lambda_use_self();
			break;
		case GDScriptParser::IdentifierNode::MEMBER_VARIABLE:
			mark_lambda_use_self();
			p_identifier->variable_source->usages++;
			[[fallthrough]];
		case GDScriptParser::IdentifierNode::STATIC_VARIABLE:
		case GDScriptParser::IdentifierNode::LOCAL_VARIABLE:
			p_identifier->set_datatype(p_identifier->variable_source->get_datatype());
			found_source = true;
			break;
		case GDScriptParser::IdentifierNode::LOCAL_ITERATOR:
			p_identifier->set_datatype(p_identifier->bind_source->get_datatype());
			found_source = true;
			break;
		case GDScriptParser::IdentifierNode::LOCAL_BIND: {
			GDScriptParser::DataType result = p_identifier->bind_source->get_datatype();
			result.is_constant = true;
			p_identifier->set_datatype(result);
			found_source = true;
		} break;