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/syntheticExtensions/samAdapters/ReturnTypeAnnotation.fir.kt

@@ -1,7 +1,7 @@
 // FILE: KotlinFile.kt
 fun foo(javaInterface: JavaInterface) {
     val value = javaInterface.compute { "" }
-    value.<!INAPPLICABLE_CANDIDATE!>length<!>
+    value.<!UNSAFE_CALL!>length<!>
 }
 
 // FILE: JavaInterface.java