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/smartCasts/enumEntryMembers_before.fir.kt

@@ -12,7 +12,7 @@ fun printMessages() {
     Message.HELLO.text!!
     Message.HELLO.text.length
 
-    Message.NOTHING.text.<!INAPPLICABLE_CANDIDATE!>length<!>
+    Message.NOTHING.text.<!UNSAFE_CALL!>length<!>
 
     Message.NOTHING.text!!
     Message.NOTHING.text.length