[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/expressions/sam/genericSamSmartcast.fir.kt.txt

@@ -1,9 +1,9 @@
 fun f(x: Any): String {
   when {
-    x is A<*> -> return x /*as A<*> */.call(block = local fun <anonymous>(y: Any?): String? {
+    x is A<*> -> return x /*as A<*> */.call(block = local fun <anonymous>(y: Any?): @FlexibleNullability String? {
       return "OK"
     }
- /*-> I<Any?>? */) /*!! String */
+ /*-> @FlexibleNullability I<@FlexibleNullability Any?>? */) /*!! @FlexibleNullability String */
   }
   return "Fail"
 }