Reject fake TypeInfos for TypedDict fallbacks

[ilevkivskyi]

This avoids possible crashes during calculations of meets and joins on a cold update.

We do this by rejecting a fake ad-hoc TypeInfo created by fixup.py if a file containing the typed dict fallback was deleted, and instead using the generic typing._TypedDict fallback.

[ilevkivskyi]

@JukkaL if you think this is relatively safe, then we can merge this in release branch, otherwise I think it is not critical because this is kind of an old issue (see e.g. the second test).

[JukkaL]

Thanks for the quick fix! This looks safe enough to cherry-pick, but it's up to you. I agree that it doesn't look urgent.

Is a similar issue possible with named tuples?

[ilevkivskyi]

I played more with the named tuples and they seem safe (mostly because fallbacks are computed on the fly, but also several places are guarded with if foo.tuple_type is not None).