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/ownerDeclaresBothModifies.fir.kt

@@ -9,5 +9,5 @@ fun foo(arg: Int?) {
     }
     if (x != null) x = 42
     // Unsafe because of lambda
-    x.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+    x.<!UNSAFE_CALL!>hashCode<!>()
 }