[K/N][IR] Never consider a enum a final class

Unfortunately, neither descriptors nor Lazy IR have enough information
to correctly deduce a enum's modality as psi2ir does, so to avoid the RTTI generator
generating different vtables in different setups (whether a klib is cached or not),
conservatively consider all enums non-final.

kotlin-native/backend.native/tests/codegen/enum/openMethodNoOverrides_main.out

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