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/varnotnull/setNullInTryUnsound.fir.kt

@@ -6,5 +6,5 @@ fun foo() {
     try {
         s = null
     } catch (ex: Exception) {}
-    s.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+    s.<!UNSAFE_CALL!>hashCode<!>()
 }