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/variables/lambdaBetweenArguments.fir.kt

@@ -6,5 +6,5 @@ fun foo(x: Int, f: () -> Unit, y: Int) {}
 fun bar() {
     var x: Int?
     x = 4
-    foo(x, { x = null; x.<!INAPPLICABLE_CANDIDATE!>hashCode<!>() }, x)
+    foo(x, { x = null; x.<!UNSAFE_CALL!>hashCode<!>() }, x)
 }