[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/javaStaticScopeInheritance.fir.txt

@@ -1,4 +1,4 @@
 FILE: main.kt
     public final fun main(): R|kotlin/Unit| {
-        lval b: R|ft<@FlexibleNullability B, B?>!| = Q|B|.R|/B.VALUE|
+        lval b: R|ft<B, B?>!| = Q|B|.R|/B.VALUE|
     }