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/varnotnull/nestedWhile.fir.kt

@@ -12,7 +12,7 @@ fun foo(): Bar {
     var y: Bar? = Bar()
     while (x != null) {
         // Here call is unsafe because of initialization and also inner loop
-        y.<!INAPPLICABLE_CANDIDATE!>next<!>()
+        y.<!UNSAFE_CALL!>next<!>()
         while (y != null) {
             if (x == y)
                 // x is not null because of outer while