[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/resolveWithStdlib/j+k/KJKInheritanceGeneric.fir.txt

@@ -16,7 +16,7 @@ FILE: K2.kt
         }
 
         public final fun bar(): R|kotlin/Unit| {
-            this@R|/K2|.R|SubstitutionOverride</J1.foo: R|ft<@FlexibleNullability kotlin/Int, kotlin/Int?>!|>|(Int(1))
+            this@R|/K2|.R|SubstitutionOverride</J1.foo: R|ft<kotlin/Int, kotlin/Int?>!|>|(Int(1))
             this@R|/K2|.R|/J1.baz|()
         }