Fix crash on invalid recursive variadic alias (#21572)
Fixes https://github.com/python/mypy/issues/21125
This one was tricky. This is because the above issue actually exposed
_two_ different crash scenarios:
* A crash on invalid constructs like `*tuple[Ts]` (must be
`*tuple[*Ts]`).
* An infinite recursion when trying to detect pathological and divergent
aliases.
And while working on this I discovered two more cases:
* A crash where an invalid type (like a union) appears in unpack in a
recursive alias definition.
* A crash on non-normalizeable recursive tuple.
I fix the first by tightening logic in `typeanal.py` w.r.t. where
exactly a `TypeVarTuple` is allowed. I fix the second and third by
avoiding `get_proper_type()` calls in `expand_type()` for recursive
tuples. The fourth is the most problematic, and is kind of a fundamental
thing. This PR only avoids an immediate crash for such aliases. We will
still need to update various call sites where we special-case tuples to
expect non-normal ones.
Couple more related things:
* I fix couple issues with `is_recursive` cache invalidation.
* I added a fast path to `detect_diverging_alias()` to avoid creating
sets unless really needed.
