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ee51c2a3 - [red-knot] fix ordering of `ClassDef` semantic index building (#16915)

[red-knot] fix ordering of `ClassDef` semantic index building (#16915) ## Summary From #16861 This PR fixes the incorrect `ClassDef` handling of `SemanticIndexBuilder::visit_stmt`, which fixes some of the incorrect behavior of referencing the class itself in the class scope (a complete fix requires a different fix, which will be done in the another PR). --------- Co-authored-by: Carl Meyer <carl@astral.sh>