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/fir/analysis-tests/testData/resolve/smartcasts/orInWhenBranch.kt

@@ -22,6 +22,6 @@ fun test_3(a: Any?, b: Boolean) {
 
 fun test_4(a: Any?, b: Boolean) {
     if (a is String || b) {
-        a.<!INAPPLICABLE_CANDIDATE{LT}!><!INAPPLICABLE_CANDIDATE{PSI}!>foo<!>()<!> // Should be Bad
+        a.<!UNSAFE_CALL{LT}!><!UNSAFE_CALL{PSI}!>foo<!>()<!> // Should be Bad
     }
 }