Fix various bugs in TypeVarTuples with defaults (#21544)
Fixes a bunch of TODOs in tests.
Fixes https://github.com/python/mypy/issues/20449
Couple non-trivial things here:
First, I prohibit (regular) type variables with defaults that appear
after a `TypeVarTuple`. The PEP and the spec are clear that these are
ambiguous and invalid. But, because they are ambiguous, we can't simply
give the error, since otherwise various parts of code will behave in
weirdly inconsistent ways. Therefore, I completely erase ambiguous type
variables out of existence, 1984 style.
Second, couple commented-out tests cases suggested that `TypeVarTuple`
defaults should be used in cases where `*tuple[()]` would be used as a
value otherwise, plus `Foo[X, Y]` and `Foo[X, Y, *tuple[()]]` should be
interpreted differently. I don't see anything about this in the
PEP/spec, and I don't like this. I think the default should be used
_only_ in cases where `Any` or `Never` would be used otherwise.
