[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/coercionToUnit.fir.kt.txt

@@ -11,14 +11,14 @@ fun test2(mc: MutableCollection<String>) {
 
 fun test3() {
   { // BLOCK
-    val tmp0_safe_receiver: PrintStream? = #out
+    val tmp0_safe_receiver: @FlexibleNullability PrintStream? = #out
     when {
       EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
       else -> tmp0_safe_receiver.println(p0 = "Hello,")
     }
   } /*~> Unit */
   { // BLOCK
-    val tmp1_safe_receiver: PrintStream? = #out
+    val tmp1_safe_receiver: @FlexibleNullability PrintStream? = #out
     when {
       EQEQ(arg0 = tmp1_safe_receiver, arg1 = null) -> null
       else -> tmp1_safe_receiver.println(p0 = "world!")