Check for overlapping overloads

[dhruvmanila]

This requires some discussion on what the concrete rules are. There's some context on a discussion about this when the overloads chapter in the typing spec was being updated: python/typing#1839 (comment).

For example (copied from the above discussion):

from typing import Literal, overload

@overload
def is_one(x: Literal[1]) -> True: ...
@overload
def is_one(x: int) -> False: ...

def f(x: int) -> bool:
    return is_one(x)

f(1)  # Presumably True at runtime, but type checker will say False (unless it performs inlining)