Improve type inference for recursive functions

[maflAT]

Currently, the return type of a function that may return the result of a recursive call, will always be inferred as
Unknown | type_of_known_return_paths.

For example, the return type of the following function is inferred as Unknown | None:

def recurse(i: int):
    if i > 0:
        return recurse(i - 1)

If I'm not mistaken, if the types of all return paths are either known (in this case it's just None) or are the result of a recursive call, then the only other possible return type would be NoReturn.

Therefore, we could narrow the return type of the above function to a more precise NoReturn | None.