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/objectLiterals/kt7110.fir.kt

@@ -7,5 +7,5 @@ abstract class A(val s: String) {
 fun foo(o: String?): Int {
     val a = object : A(o!!){}
     a.bar()
-    return o.<!INAPPLICABLE_CANDIDATE!>length<!>
+    return o.<!UNSAFE_CALL!>length<!>
 }