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/controlStructures/forLoopWithNullableRange.fir.kt

@@ -8,7 +8,7 @@ class It {
 }
 
 fun test(c: Coll?) {
-  <!INAPPLICABLE_CANDIDATE!>for (x in c) {}<!>
+  <!UNSAFE_CALL!>for (x in c) {}<!>
 
   if (c != null) {
     for(x in c) {}