- 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
Before this commit, we cached such IR properties by FIR property
which was created by Java field each time when we referenced it.
This led to signature clashes. Now we cache such IR properties
directly by associated FIR field.