[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/samConversionsWithSmartCasts.fir.kt.txt

@@ -18,7 +18,7 @@ fun test3(a: Function0<Unit>) {
 
 fun test4(a: Function0<Unit>, b: Function0<Unit>) {
   when {
-    a is Runnable -> J().run2(r1 = a /*as Runnable */, r2 = b /*-> Runnable? */)
+    a is Runnable -> J().run2(r1 = a /*as Runnable */, r2 = b /*-> @FlexibleNullability Runnable? */)
   }
 }
 
@@ -39,19 +39,19 @@ fun test5x(a: Any) {
 
 fun test6(a: Any) {
   a as Function0<Unit> /*~> Unit */
-  J().run1(r = a /*as Function0<Unit> */ /*-> Runnable? */)
+  J().run1(r = a /*as Function0<Unit> */ /*-> @FlexibleNullability Runnable? */)
 }
 
 fun test7(a: Function1<Int, Int>) {
   a as Function0<Unit> /*~> Unit */
-  J().run1(r = a /*as Function0<Unit> */ /*-> Runnable? */)
+  J().run1(r = a /*as Function0<Unit> */ /*-> @FlexibleNullability Runnable? */)
 }
 
 fun test8(a: Function0<Unit>) {
-  J().run1(r = id<Function0<Unit>?>(x = a /*-> Function0<Unit>? */) /*-> Runnable? */)
+  J().run1(r = id<@FlexibleNullability Function0<Unit>?>(x = a /*-> @FlexibleNullability Function0<Unit>? */) /*-> @FlexibleNullability Runnable? */)
 }
 
 fun test9() {
-  J().run1(r = ::test9 /*-> Runnable? */)
+  J().run1(r = ::test9 /*-> @FlexibleNullability Runnable? */)
 }