[red-knot] detect unreachable attribute assignments (#16852)
## Summary
This PR closes #15967.
Attribute assignments that are statically known to be unreachable are
excluded from consideration for implicit instance attribute type
inference. If none of the assignments are found to be reachable, an
`unresolved-attribute` error is reported.
## Test Plan
[A test
case](https://github.com/astral-sh/ruff/blob/main/crates/red_knot_python_semantic/resources/mdtest/attributes.md#attributes-defined-in-statically-known-to-be-false-branches)
marked as TODO now work as intended, and new test cases have been added.
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Co-authored-by: David Peter <mail@david-peter.de>
