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/varnotnull/assignment.fir.kt

@@ -1,11 +1,11 @@
 // !WITH_NEW_INFERENCE
 fun foo() {
     var v: String? = null
-    v.<!INAPPLICABLE_CANDIDATE!>length<!>
+    v.<!UNSAFE_CALL!>length<!>
     v = "abc"
     v.length
     v = null
-    v.<!INAPPLICABLE_CANDIDATE!>length<!>
+    v.<!UNSAFE_CALL!>length<!>
     v = "abc"
     v.length
 }