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/alwaysNull.fir.kt

@@ -3,7 +3,7 @@ fun foo(): String {
     var s: String?
     s = null
     s?.length
-    s.<!INAPPLICABLE_CANDIDATE!>length<!>
+    s.<!UNSAFE_CALL!>length<!>
     if (s == null) return s!!
     var t: String? = "y"
     if (t == null) t = "x"