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/fir/analysis-tests/testData/resolve/smartcasts/variables/delayedAssignment.kt

@@ -11,5 +11,5 @@ fun test(b: Boolean) {
     } else {
         a = null
     }
-    a.<!INAPPLICABLE_CANDIDATE{LT}!><!INAPPLICABLE_CANDIDATE{PSI}!>foo<!>()<!>
+    a.<!UNSAFE_CALL{LT}!><!UNSAFE_CALL{PSI}!>foo<!>()<!>
 }