[red-knot] Check overloads without an implementation (#17681)
## Summary
As mentioned in the spec
(https://typing.python.org/en/latest/spec/overload.html#invalid-overload-definitions),
part of #15383:
> The `@overload`-decorated definitions must be followed by an overload
implementation, which does not include an `@overload` decorator. Type
checkers should report an error or warning if an implementation is
missing. Overload definitions within stub files, protocols, and on
abstract methods within abstract base classes are exempt from this
check.
## Test Plan
Remove TODOs from the test; create one diagnostic snapshot.
