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/nestedLoopsWithLongBreak.fir.kt

@@ -13,7 +13,7 @@ public fun foo(p: String?, r: String?, q: String?): Int {
         if (!x(p)) break
     }
     // Long break allows r == null
-    r.<!INAPPLICABLE_CANDIDATE!>length<!>
+    r.<!UNSAFE_CALL!>length<!>
     // Smart cast is possible
     q.length
     return p.length