FIR checker: differentiate UNSAFE_CALL from INAPPLICABLE_CANDIDATE

To do so, inside the root cause of inapplicable candidate errors,
we will record expected/actual type of receiver, if any.
That will help identifying inapplicable calls on nullable receiver.

compiler/testData/diagnostics/tests/smartCasts/elvis/basicOff.fir.kt

@@ -11,13 +11,13 @@ interface Order {
 fun foo(o: Any) {
     val order = o as? Order
     if (order?.expired ?: false) {
-        order.<!INAPPLICABLE_CANDIDATE!>doSomething<!>()
+        order.<!UNSAFE_CALL!>doSomething<!>()
     }
     else {
 
     }
     if (order?.notExpired() ?: false) {
-        order.<!INAPPLICABLE_CANDIDATE!>doSomething<!>()
+        order.<!UNSAFE_CALL!>doSomething<!>()
     }
 }
 
@@ -39,12 +39,12 @@ fun bar(o: Any) {
 
 fun baz(o: Boolean?) {
     if (o ?: false) {
-        o.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+        o.<!UNSAFE_CALL!>hashCode<!>()
     }
     if (o ?: true) {
 
     }
     else {
-        o.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+        o.<!UNSAFE_CALL!>hashCode<!>()
     }
 }