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/Nullability.fir.kt

@@ -158,7 +158,7 @@ fun test() {
 
 fun f(out : String?) {
   out?.get(0)
-  out.<!INAPPLICABLE_CANDIDATE!>get<!>(0)
+  out.<!UNSAFE_CALL!>get<!>(0)
   if (out != null) else return;
   out.get(0)
 }