Variants pointer to object can unadvertedly become invalid

[RandomShaper]

Godot version:
All of them? (At least 3.1.x and 3.2.x.)

OS/device including version:
N/A

Issue description:

The problem is the way Godot manages the memory of non-Resource objects. Since they are not reference-counted, you can have Variants with dangling pointers to freed ones. This is a big problem, because it affects scripting.

In addition, if a newly created object happens to be at the same address of a freed one, any dangling pointer to the old one will be effectively pointing to the new one.

Steps to reproduce:

This simple code is able to demonstrate the problem:

func _ready():
	var a = []
	var c = MarginContainer.new()
	a.append(c)
	print(a[0])
	c.free()
	print(a[0])
	TextureRect.new()
	print(a[0])

Output:

[MarginContainer:1171]
[Deleted Object]
[TextureRect:1172]

As can be seen, the TextureRect has been allocated at the same address the MarginContainer was.

Minimal reproduction project:

Not needed.

[RandomShaper]

Duplicate of #32383.