[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/ir/irText/expressions/sam/samConversionInGenericConstructorCall.fir.kt.txt

@@ -1,8 +0,0 @@
-fun test1(f: Function1<String, String>): C<String?> {
-  return C<String?>(jxx = f /*-> J<String?, String?>? */)
-}
-
-fun test2(x: Any) {
-  x as Function1<String, String> /*~> Unit */
-  C<String?>(jxx = x /*as Function1<String, String> */ /*-> J<String?, String?>? */) /*~> Unit */
-}