In some cases we transform flexible type into non-flexible during
enhancement, but don't add `@EnhancedNullability` attribute to them,
which breaks consistency with K1 on IR level
This commit fixes it
^KT-65302
- Add utilities to add new attribute to ConeAttributes
- Get rid of FlexibleNullability attribute (it can be easily inferred
for any flexible type at any moment)
- Fix determining of EnhancedNullability attribute
Dmitriy Novozhilov