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/testsWithStdLib/contracts/smartcasts/extensionReceiver.fir.kt

@@ -17,7 +17,7 @@ fun CharSequence?.isNullOrEmpty(): Boolean {
 fun smartcastOnReceiver(s: String?) {
     with(s) {
         if (isNullOrEmpty()) {
-            <!INAPPLICABLE_CANDIDATE!>length<!>
+            <!UNSAFE_CALL!>length<!>
         }
         else {
             length
@@ -32,7 +32,7 @@ fun mixedReceiver(s: String?) {
         }
     } else {
         with(s) {
-            <!INAPPLICABLE_CANDIDATE!>length<!>
+            <!UNSAFE_CALL!>length<!>
         }
     }
 }