[FIR] Resolve elvis call as special synthetic call

Before that commit we desugared `a ?: b` as

when (val elvis = a) {
    null -> b
    else -> elvis
}

It was incorrect, because `a` should be resolved in dependent mode,
  but when it was `elvis` initializer it was resolved in independent
  mode, so we can't infer type for `a` in some complex cases

compiler/testData/diagnostics/tests/inference/nothingType/discriminatedNothingAndSmartCast.fir.kt

@@ -15,5 +15,5 @@ fun test(s: String?, silent: Boolean) {
             throw Exception()
         }
 
-    s.<!INAPPLICABLE_CANDIDATE!>length<!>
+    s.length
 }