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/dataFlow/assignment/uninitializedValNullability.fir.kt

@@ -10,6 +10,6 @@ fun test(a: Any?, flag: Boolean, x: Any?) {
     }
     else {
         b = x
-        b.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+        b.<!UNSAFE_CALL!>hashCode<!>()
     }
 }