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/properties/inferenceFromGetters/primaryConstructorParameter.fir.kt

@@ -19,5 +19,5 @@ class C(p: Any, val v: Any) {
 
     var test5
         get() { return <!UNRESOLVED_REFERENCE!>p<!> }
-        set(nv) { <!UNRESOLVED_REFERENCE!>p<!>.<!INAPPLICABLE_CANDIDATE!>let<!> {} }
+        set(nv) { <!UNRESOLVED_REFERENCE!>p<!>.<!UNSAFE_CALL!>let<!> {} }
 }