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/tests-spec/testData/diagnostics/notLinked/dfa/pos/52.fir.kt

@@ -89,7 +89,7 @@ fun <T : String?> T.case_9() = this
 fun case_9() {
     var x: Int? = 10
     x = null
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Int?")!>x<!>.<!INAPPLICABLE_CANDIDATE!>case_9<!>()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Int?")!>x<!>.<!UNSAFE_CALL!>case_9<!>()
 }
 
 /*