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/smartcastAfterReassignment.kt

@@ -18,5 +18,5 @@ fun test_3() {
     x = ""
     x.length
     x = null
-    x.<!INAPPLICABLE_CANDIDATE!>length<!>
+    x.<!UNSAFE_CALL!>length<!>
 }