Try to guess method return type even if subscript base has no value

[HolonProduction]

Fixes #78003.

We want to guess the method return type not only if the base has a reduced value but also if the base has a resolved type.

The following example shows the problem:

class B:
	func to_str(b: int):
		return str(b)

var a: B

func _ready():
	a.to_str(10) # Godot does not suggest string methods in autocompletion

Since a has no value in the example, the autocompletion does not try to guess the type of to_str(10).

Note that we need to use internal classes to observe that this PR solves the problem. There are other problems preventing it to work in the context of #78003.

[HolonProduction]

Rebased and added a test case

[Mickeon]

I'm approving on principle. Doing this speeds up... a general workflow where writing code faster is a priority and polishing it with static typing is an afterthought. In other words, it's just nice.

[akien-mga]

Let's give this a try!

[akien-mga]

Might use a rebase though, there's been changes to the test suite in recent months so we should make sure it still works properly.

[HolonProduction]

Rebased and adjusted the test to account for #99102

[dalexeev]

I haven't tested, but it makes sense to me.

[akien-mga]

Thanks!