[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/fir/analysis-tests/testData/resolve/inference/capturedTypeForJavaTypeParameter.fir.txt

@@ -4,5 +4,5 @@ FILE: main.kt
     public final fun <D : R|A|> foo(b: R|B<D>|): R|kotlin/Unit| {
     }
     public final fun main(b: R|B<*>|): R|kotlin/Unit| {
-        R|/foo|<R|ft<@FlexibleNullability A, A?>!|>(R|<local>/b|)
+        R|/foo|<R|ft<A, A?>!|>(R|<local>/b|)
     }