[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/declarations/localDelegatedProperties.fir.kt.txt

@@ -12,18 +12,19 @@ fun test1() {
 }
 
 fun test2() {
-  var x: Int?
+  var x: @FlexibleNullability Int?
     val x$delegate: HashMap<String, Int> = hashMapOf<String, Int>()
-    local get(): Int? {
-      return x$delegate.getValue<Int?, Int?>(thisRef = null, property = ::x)
+    local get(): @FlexibleNullability Int? {
+      return x$delegate.getValue<@FlexibleNullability Int?, @FlexibleNullability Int?>(thisRef = null, property = ::x)
     }
-    local set(<set-?>: Int?) {
-      x$delegate.setValue<Int?>(thisRef = null, property = ::x, value = <set-?>)
+    local set(<set-?>: @FlexibleNullability Int?) {
+      x$delegate.setValue<@FlexibleNullability Int?>(thisRef = null, property = ::x, value = <set-?>)
     }
 
   <set-x>(<set-?> = 0)
-  val <unary>: Int? = <get-x>()
+  val <unary>: @FlexibleNullability Int? = <get-x>()
   <set-x>(<set-?> = <unary>.inc())
   <unary> /*~> Unit */
   <set-x>(<set-?> = <get-x>().plus(other = 1))
 }
+