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/leftElvisBreakInsideWhileTrue.fir.kt

@@ -7,5 +7,5 @@ public fun foo(x: String?, y: String?): Int {
         y.length
     }
     // y is null because of the break
-    return y.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return y.<!UNSAFE_CALL!>length<!>
 }