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/testsWithStdLib/smartcasts/withChangesToNull.fir.kt

@@ -2,7 +2,7 @@ fun foo(y: String?) {
     var x: String? = ""
     if (x != null) {
         with(y?.let { x = null; it }) {
-            this.<!INAPPLICABLE_CANDIDATE!>length<!>
+            this.<!UNSAFE_CALL!>length<!>
             x.length
         }
         x.length