Tighten overload impl return type

```py
@overload
def foo(a: int) -> int: ...
@overload
def foo(a: str) -> str: ...
def foo(a: object) -> object:
return object()
```

I think that the return type shouldn't be completely unchecked, it should be a union of the overloads return types or narrower.

The true form of the return type _isn't_ an intersection, it's actually wider than that. It's a conditional type based on the overloads.

No files or tests are named. Start by locating the overload implementation return-type checking, then determine how overload return types should constrain the implementation and how conditional behavior is represented; done means incompatible implementation returns are rejected while valid narrower returns remain accepted.

Written by the indexing model from the issue text.