[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/collections/kt44233.kt

@@ -1,5 +1,4 @@
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
 // FULL_JDK
 
 import java.util.concurrent.ConcurrentSkipListSet
@@ -15,4 +14,4 @@ fun box(): String {
         "Failed"
     else
         "OK"
-}
+}

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