Further fix overload diagnostic for vararg and varkwarg

[hauntsaninja]

#19614 got merged faster than I expected :-)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ilevkivskyi]

meet can often produce <Never>, but in this case I guess it is OK.

[hauntsaninja]

Yeah, in this case I think it is desirable. If the meet is Never then I think it does mean that implementation would have wrong types depending on how it is called:

@overload
def foo(x: int) -> None: ...

def foo(*args: int, **kw: str) -> None:
    pass