[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/BasicWithAnnotatedOverriddenJava.fir.txt

@@ -5,8 +5,8 @@ FILE: jvm.kt
         }
 
         public final fun test(): R|kotlin/Unit| {
-            lval x: R|@FlexibleNullability kotlin/String| = this@R|/User|.R|/AnnotatedDerived.foo|(String(123))
-            lval y: R|@FlexibleNullability kotlin/String| = this@R|/User|.R|/AnnotatedDerived.foo|(Null(null))
+            lval x: R|kotlin/String| = this@R|/User|.R|/AnnotatedDerived.foo|(String(123))
+            lval y: R|kotlin/String| = this@R|/User|.R|/AnnotatedDerived.foo|(Null(null))
         }
 
     }