[FIR] Set EnhancedNullability attribute for former flexible types

In some cases we transform flexible type into non-flexible during
  enhancement, but don't add `@EnhancedNullability` attribute to them,
  which breaks consistency with K1 on IR level

This commit fixes it

^KT-65302

compiler/testData/ir/irText/classes/delegatedImplementationOfJavaInterface.fir.kt.txt

@@ -15,7 +15,7 @@ class Test : J {
   }
 
   @Nullable
-  override fun returnNullable(): String? {
+  override fun returnNullable(): @EnhancedNullability String? {
     return <this>.#j.returnNullable()
   }
 
@@ -31,7 +31,7 @@ class Test : J {
     <this>.#j.takeNotNull(x = x)
   }
 
-  override fun takeNullable(@Nullable x: String?) {
+  override fun takeNullable(@Nullable x: @EnhancedNullability String?) {
     <this>.#j.takeNullable(x = x)
   }