[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/diagnostics/testsWithStdLib/java/functionN.fir.kt

@@ -47,7 +47,7 @@ fun main() {
     a.baz(listOf())
 
     a.manyParams(null)
-    a.manyParams(any<kotlin.jvm.functions.FunctionN<Unit>>())
+    a.<!NONE_APPLICABLE!>manyParams<!>(any<kotlin.jvm.functions.FunctionN<Unit>>())
 
     // Potentially, this would have better to forbid calling manyParams, too.
     // But it might be complicated because we need to match that it is an override