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/testsWithStdLib/smartcasts/letChangesToNullComplex.fir.kt

@@ -5,7 +5,7 @@ fun bar(z: String?) = z
 fun foo(y: String?) {
     var x: String? = ""
     if (x != null) {
-        bar(y?.let { x = null; it }).<!INAPPLICABLE_CANDIDATE!>length<!>
+        bar(y?.let { x = null; it }).<!UNSAFE_CALL!>length<!>
         x.length // Smart cast is not possible
     }
 }