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/contracts/fromStdlib/isNullOrEmpty.fir.kt

@@ -3,7 +3,7 @@
 
 fun testIsNullOrEmpty(x: String?) {
     if (x.isNullOrEmpty()) {
-        x.<!INAPPLICABLE_CANDIDATE!>length<!>
+        x.<!UNSAFE_CALL!>length<!>
     }
     else {
         x.length
@@ -15,6 +15,6 @@ fun testIsNotNullOrEmpty(x: String?) {
         x.length
     }
 
-    x.<!INAPPLICABLE_CANDIDATE!>length<!>
+    x.<!UNSAFE_CALL!>length<!>
 }