Fix type variable defaults depending on previous variables (#21526)

Fixes https://github.com/python/mypy/issues/19192
Fixes https://github.com/python/mypy/issues/20027
Fixes few TODOs in tests (don't know if there are relevant issues)
Closes https://github.com/python/mypy/pull/19382

Core idea is simple: don't do bizarre things :-) More precisely, instead
of various in-place modifications (which is a big no-no for types),
simply expand defaults iteratively as we build the environment for the
type. Couple additional things:
* Update defaults in `freshen_function_type_vars()` this is required for
consistency when updating `TypeVarId`s.
* Use `fix_instance()` in `checkexpr.py` for type applications. This
will apply _all_ type arguments in cases like `x = Foo[int](...)`. The
PEP and the spec say this is how it should be.