Fix crash on invalid redefinition in dataclass

[ilevkivskyi]

Fixes #20476

Fix seems to be quite simple: if there are some weird redefinitions, just ignore them, an error is already given.

[ilevkivskyi]

Both errors in homeassistant are correct (inconsistent self-binding), but the error message in the first one is really confusing. I will check if it is easy to fix.

[ilevkivskyi]

Actually, I think the second one is wrong, we should expand the self-type there.

[ilevkivskyi]

OK, the situation is quite messy. So I will try to simply restore the existing behavior.

[ilevkivskyi]

After even more thinking, the dataclasses are different from regular classes in how they handle callables. So let's not go down this rabbit hole, and focus on simply fixing the crash.

I will however keep all the tests fore future reference.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
- tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports