[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/modality/ModalityOfFakeOverrides.fir.txt

@@ -1,4 +1,4 @@
-public open class ModalityOfFakeOverrides : R|java/util/AbstractList<ft<@FlexibleNullability kotlin/String, kotlin/String?>!>| {
+public open class ModalityOfFakeOverrides : R|java/util/AbstractList<ft<kotlin/String, kotlin/String?>!>| {
     @R|java/lang/Override|() @R|org/jetbrains/annotations/NotNull|() public open operator fun get(index: R|kotlin/Int|): R|@EnhancedNullability kotlin/String|
 
     public constructor(): R|test/ModalityOfFakeOverrides|