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/regressions/kt12898.fir.kt

@@ -10,7 +10,7 @@ class C(override val t: Any?) : B<Any?, Any>
 fun f(b: B<*, Any>) {
     val y = b.t
     if (y is String?) {
-        y.<!INAPPLICABLE_CANDIDATE!>length<!>
+        y.<!UNSAFE_CALL!>length<!>
     }
 }