[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/samConstructors.fir.kt.txt

@@ -16,9 +16,10 @@ fun test3(): Runnable {
   return ::foo /*-> Runnable */
 }
 
-fun test4(): Comparator<Int?> {
-  return local fun <anonymous>(a: Int?, b: Int?): Int {
-    return a.minus(other = b /*!! Int */)
+fun test4(): Comparator<@FlexibleNullability Int?> {
+  return local fun <anonymous>(a: @FlexibleNullability Int?, b: @FlexibleNullability Int?): Int {
+    return a.minus(other = b /*!! @FlexibleNullability Int */)
   }
- /*-> Comparator<Int?> */
+ /*-> Comparator<@FlexibleNullability Int?> */
 }
+