[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/firProblems/SameJavaFieldReferences.fir.kt.txt

@@ -1,4 +1,4 @@
 fun foo() {
-  val ref1: KProperty0<String?> = SomeJavaClass::someJavaField/*</* null */>()*/
-  val ref2: KProperty0<String?> = SomeJavaClass::someJavaField/*</* null */>()*/
+  val ref1: KProperty0<@FlexibleNullability String?> = SomeJavaClass::someJavaField/*</* null */>()*/
+  val ref2: KProperty0<@FlexibleNullability String?> = SomeJavaClass::someJavaField/*</* null */>()*/
 }