[FIR] Fix enhancement of FlexibleNullability and EnhancedNullability

- 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

compiler/testData/loadJava/compiledJava/sam/SubstitutedSamInterface.fir.txt

@@ -1,2 +1,2 @@
-public abstract interface SubstitutedSamInterface : R|java/util/Comparator<ft<@FlexibleNullability kotlin/String, kotlin/String?>!>| {
+public abstract interface SubstitutedSamInterface : R|java/util/Comparator<ft<kotlin/String, kotlin/String?>!>| {
 }