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/resolve/invoke/invokeAndSmartCast.fir.kt

@@ -7,8 +7,8 @@ fun test(a: A) {
         (a.x)("")
     }
     "".<!UNRESOLVED_REFERENCE!>(a.x)()<!>
-    a.<!INAPPLICABLE_CANDIDATE!>x<!>("")
-    <!INAPPLICABLE_CANDIDATE!>(a.x)("")<!>
+    a.<!UNSAFE_CALL!>x<!>("")
+    <!UNSAFE_CALL!>(a.x)("")<!>
 
     with("") {
         a.<!INAPPLICABLE_CANDIDATE!>x<!>()