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/tests-spec/testData/diagnostics/notLinked/dfa/neg/23.fir.kt

@@ -61,7 +61,7 @@ inline fun <reified T, reified K> case_6() {
     var x: T? = 10 as T
     if (x is K) {
         x = null
-        <!DEBUG_INFO_EXPRESSION_TYPE("T? & T?")!>x<!>.<!INAPPLICABLE_CANDIDATE!>equals<!>(10)
+        <!DEBUG_INFO_EXPRESSION_TYPE("T? & T?")!>x<!>.<!UNSAFE_CALL!>equals<!>(10)
         <!DEBUG_INFO_EXPRESSION_TYPE("T? & T?")!>x<!>
         println(1)
     }