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/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromLibrary/notIsNullOrEmpty.kt

@@ -7,7 +7,7 @@ fun test_1(s: String?) {
 fun test_2(s: String?) {
     // contracts related
     if (s.isNullOrEmpty()) {
-        s.<!INAPPLICABLE_CANDIDATE!>length<!> // Should be bad
+        s.<!INAPPLICABLE_CANDIDATE{LT}, UNSAFE_CALL!>length<!> // Should be bad
     } else {
         s.length // Should be OK
     }