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/extensionSafeCall.fir.kt

@@ -9,5 +9,5 @@ fun <T> T?.let(f: (T) -> Unit) {
 fun test(your: Your?) {
     (your?.foo() as? Any)?.let {}
     // strange smart cast to 'Your' at this point
-    your.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+    your.<!UNSAFE_CALL!>hashCode<!>()
 }