[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/ir/irText/stubs/javaSyntheticProperty.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DUMP_DEPENDENCIES
 // FILE: javaSyntheticProperty.kt
 val test = J().foo