Fix overlap check for variadic generics (#18638)
Fixes https://github.com/python/mypy/issues/18105
When I implemented this initially, I only handled tuples, but forgot
instances, thus causing the crash. I don't add many tests, since the
instance overlap check simply relays to the tuple one, that is already
tested.
(Btw I already forgot how verbose everything is in the `TypeVarTuple`
world :-))
