[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/samConversions.fir.kt.txt

@@ -1,29 +0,0 @@
-fun J.test0(a: Runnable) {
-  runStatic(r = a)
-  <this>.runIt(r = a)
-}
-
-fun test1() {
-  runStatic(r = local fun <anonymous>() {
-    test1()
-  }
- /*-> Runnable? */)
-}
-
-fun J.test2() {
-  <this>.runIt(r = local fun <anonymous>() {
-    test1()
-  }
- /*-> Runnable? */)
-}
-
-fun J.test3(a: Function0<Unit>) {
-  <this>.run2(r1 = a /*-> Runnable? */, r2 = a /*-> Runnable? */)
-}
-
-fun J.test4(a: Function0<Unit>, b: Function0<Unit>, flag: Boolean) {
-  <this>.runIt(r = when {
-    flag -> a
-    else -> b
-  } /*-> Runnable? */)
-}