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/varnotnull/varNull.fir.kt

@@ -2,5 +2,5 @@
 fun foo(): Int {
     var s: String? = "abc"
     s = null
-    return s.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return s.<!UNSAFE_CALL!>length<!>
 }