[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/classes/delegatedImplementationOfJavaInterface.fir.kt.txt

@@ -9,11 +9,11 @@ class Test : J {
     <this>.#<$$delegate_0>.takeNotNull(x = x)
   }
 
-  override fun takeNullable(x: @FlexibleNullability String?) {
+  override fun takeNullable(x: @EnhancedNullability String?) {
     <this>.#<$$delegate_0>.takeNullable(x = x)
   }
 
-  override fun takeFlexible(x: String?) {
+  override fun takeFlexible(x: @FlexibleNullability String?) {
     <this>.#<$$delegate_0>.takeFlexible(x = x)
   }
 
@@ -23,11 +23,11 @@ class Test : J {
   }
 
   @Nullable
-  override fun returnNullable(): @FlexibleNullability String? {
+  override fun returnNullable(): @EnhancedNullability String? {
     return <this>.#<$$delegate_0>.returnNullable()
   }
 
-  override fun returnsFlexible(): String? {
+  override fun returnsFlexible(): @FlexibleNullability String? {
     return <this>.#<$$delegate_0>.returnsFlexible()
   }
 
@@ -37,3 +37,4 @@ class Test : J {
     private get
 
 }
+