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/ifBreakExprInsideWhileTrue.fir.kt

@@ -6,5 +6,5 @@ public fun foo(x: String?): Int {
         y.length
     }
     // x is null because of the break
-    return x.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return x.<!UNSAFE_CALL!>length<!>
 }