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/expresssions/syntheticSmartCast.kt

@@ -34,7 +34,7 @@ fun test3(x: AnotherClass?) {
 fun test4(x: SomeClass?) {
     val bar = x?.bar
     if (bar != null) {
-        x.bar.<!INAPPLICABLE_CANDIDATE!>length<!>
+        x.bar.<!UNSAFE_CALL!>length<!>
     }
 }