GDScript classes can be replaced by anything

[Zylann]

The following code works:

	Sprite = 42
	print(Sprite)

And effectiveley replaces Sprite as if it was a normal global variable.
It looks like a bug, and a way to troll a game project really hard^^
Someone (maybe accidentally) did that in a QA thread: https://godotengine.org/qa/14991/extention-add_exception-nonexistent-function-add_exception

Might be related to #6221

[Zylann]

Still happens in Godot 3.0 stable.

[vnen]

Solved by #19264.