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/neg/45.fir.kt

@@ -9,7 +9,7 @@
 fun case_1(x: Number?): Long? {
     if (x is Long?) return x
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Number?")!>x<!>
-    return <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Number?")!>x<!>.<!INAPPLICABLE_CANDIDATE!>toLong<!>()
+    return <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Number?")!>x<!>.<!UNSAFE_CALL!>toLong<!>()
 }
 
 /*