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/fieldPlus.fir.kt

@@ -9,6 +9,6 @@ fun foo() {
     var y: My? = My(42)
     if (y!!.x != null) {
         y = My(null)
-        (y + My(0)).x.<!INAPPLICABLE_CANDIDATE!>hashCode<!>()
+        (y + My(0)).x.<!UNSAFE_CALL!>hashCode<!>()
     }
 }