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/openInSealed.fir.kt

@@ -2,7 +2,7 @@ sealed class My(open val x: Int?) {
     init {
         if (x != null) {
             // Should be error: property is open
-            x.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+            x.<!UNSAFE_CALL!>hashCode<!>()
         }
     }
 }