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/inference/kt39010.fir.kt

@@ -5,7 +5,7 @@ class A<E> {
 class B(var a: A<*>?) {
     fun bar() {
         if (a != null) {
-            a.<!INAPPLICABLE_CANDIDATE!>foo<!>()
+            a.<!UNSAFE_CALL!>foo<!>()
         }
     }
 }