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/loops/whileTrivial.fir.kt

@@ -7,5 +7,5 @@ public fun foo(p: String?): Int {
         if (x()) break
     }
     // Smart cast should not work in this case, see KT-6284
-    return p.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return p.<!UNSAFE_CALL!>length<!>
 }