[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/codegen/box/reflection/typeOf/flexibleType.kt

@@ -1,6 +1,5 @@
 // !USE_EXPERIMENTAL: kotlin.ExperimentalStdlibApi
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
 // WITH_REFLECT
 // FILE: box.kt