[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/fir/analysis-tests/testData/resolve/exhaustiveness/positive/exhaustiveWhenAndFlexibleType.fir.txt

@@ -15,7 +15,7 @@ FILE: main.kt
 
     }
     public final fun test_1(): R|kotlin/Unit| {
-        lval e: R|ft<@FlexibleNullability E, E?>!| = Q|Utils|.R|/Utils.getEnum|()
+        lval e: R|ft<E, E?>!| = Q|Utils|.R|/Utils.getEnum|()
         lval s: R|kotlin/String| = when (R|<local>/e|) {
             ==($subj$, Null(null)) ->  {
                 ^test_1 Unit
@@ -34,7 +34,7 @@ FILE: main.kt
         R|<local>/s|.R|kotlin/String.length|
     }
     public final fun test_2(): R|kotlin/Unit| {
-        lval e: R|ft<@FlexibleNullability E, E?>!| = Q|Utils|.R|/Utils.getEnum|()
+        lval e: R|ft<E, E?>!| = Q|Utils|.R|/Utils.getEnum|()
         lval s: R|kotlin/String| = when (R|<local>/e|) {
             ==($subj$, Q|E|.R|/E.A|) ->  {
                 String()