FIR: enhance in/out-variant type arguments

compiler/testData/ir/irText/firProblems/kt19251.fir.kt.txt

@@ -4,9 +4,9 @@ fun box(): String {
     return TODO()
   }
  /*-> Fun */
-  return map.computeIfAbsent(p0 = fn, p1 = local fun <anonymous>(it: @FlexibleNullability Fun?): @FlexibleNullability String? {
+  return map.computeIfAbsent(p0 = fn, p1 = local fun <anonymous>(it: Fun): String {
     return "OK"
   }
- /*-> @EnhancedNullability Function<@FlexibleNullability Fun?, @FlexibleNullability String?> */)
+ /*-> @EnhancedNullability Function<Fun, String> */)
 }