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/safecalls/innerReceiver.fir.kt

@@ -4,5 +4,5 @@ fun calc(x: String?, y: String?): Int {
     // Smart cast because of y!! in receiver
     x?.subSequence(y!!.subSequence(0, 1).length, y.length)
     // No smart cast possible
-    return y.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return y.<!UNSAFE_CALL!>length<!>
 }