[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/kt29833.fir.kt.txt

@@ -7,12 +7,13 @@ object Definitions {
 
   }
 
-  const val KT_CONSTANT: String?
+  const val KT_CONSTANT: @FlexibleNullability String?
     field = "constant"
     get
 
-  val ktValue: String?
+  val ktValue: @FlexibleNullability String?
     field = #CONSTANT
     get
 
 }
+