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/safecalls/twoArgs.fir.kt

@@ -2,5 +2,5 @@ fun calc(x: List<String>?): Int {
     // x should be non-null in arguments list
     x?.subList(x.size - 1, x.size)
     // but not also here!
-    return x.<!INAPPLICABLE_CANDIDATE!>size<!>
+    return x.<!UNSAFE_CALL!>size<!>
 }