[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/firProblems/JCTree.fir.kt.txt

@@ -13,7 +13,7 @@ class Owner<out T : JCTree> {
     get(): String {
       var tree: JCTree = <this>.<get-tree>()
       when {
-        tree /*as T */ is JCTypeApply -> return tree /*as T */ /*as JCTypeApply */.#clazz /*!! String */
+        tree /*as T */ is JCTypeApply -> return tree /*as T */ /*as JCTypeApply */.#clazz /*!! @FlexibleNullability String */
       }
       return ""
     }