[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/genericSamProjectedOut.fir.kt.txt

@@ -1,15 +1,15 @@
 fun test(a: SomeJavaClass<out String>) {
-  a.someFunction(hello = local fun <anonymous>(it: String?) {
+  a.someFunction(hello = local fun <anonymous>(it: @FlexibleNullability String?) {
     return Unit
   }
- /*-> Hello<String?>? */)
-  a.plus(hello = local fun <anonymous>(it: String?) {
+ /*-> @FlexibleNullability Hello<@FlexibleNullability String?>? */)
+  a.plus(hello = local fun <anonymous>(it: @FlexibleNullability String?) {
     return Unit
   }
- /*-> Hello<String?>? */)
-  a.get(hello = local fun <anonymous>(it: String?) {
+ /*-> @FlexibleNullability Hello<@FlexibleNullability String?>? */)
+  a.get(hello = local fun <anonymous>(it: @FlexibleNullability String?) {
     return Unit
   }
- /*-> Hello<String?>? */)
+ /*-> @FlexibleNullability Hello<@FlexibleNullability String?>? */)
 }