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/extensions/ExtensionFunctions.fir.kt

@@ -64,7 +64,7 @@ import outer.*
 
             command.foo
 
-            command.<!INAPPLICABLE_CANDIDATE!>equals<!>(null)
+            command.<!UNSAFE_CALL!>equals<!>(null)
             command?.equals(null)
             command.equals1(null)
             command?.equals1(null)