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/forWithNullableIterator.fir.kt

@@ -8,5 +8,5 @@ class It {
 }
 
 fun test() {
-  <!INAPPLICABLE_CANDIDATE, INAPPLICABLE_CANDIDATE!>for (x in Coll()) {}<!>
+  <!UNSAFE_CALL, UNSAFE_CALL!>for (x in Coll()) {}<!>
 }