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/smartCastOnWhen.fir.kt

@@ -15,6 +15,6 @@ fun bar(): Int {
     return when(ss) {
         "abc" -> ss
         else -> "xyz"
-    }.<!INAPPLICABLE_CANDIDATE!>length<!>
+    }.<!UNSAFE_CALL!>length<!>
 }