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/intersectionScope/unstableSmartCast.fir.kt

@@ -33,6 +33,6 @@ fun test() {
         x.baz(1).checkType { _<Unit>() }
         x.<!INAPPLICABLE_CANDIDATE!>baz<!>(1, 2)
 
-        x.<!UNRESOLVED_REFERENCE!>foobar<!>().<!INAPPLICABLE_CANDIDATE!>checkType<!> { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
+        x.<!UNRESOLVED_REFERENCE!>foobar<!>().<!UNSAFE_CALL!>checkType<!> { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
     }
 }