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/WhileTrueWithBreakInIfCondition.fir.kt

@@ -9,5 +9,5 @@ fun checkJump(x: Int?, y: Int?) {
         y.hashCode()
     }
     // Smart cast here is erroneous: y is nullable
-    y.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+    y.<!UNSAFE_CALL!>hashCode<!>()
 }