[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/fir/analysis-tests/testData/resolve/delegates/delegateWithLambda.txt

@@ -24,15 +24,7 @@ FILE: delegateWithLambda.kt
         }
 
         public final val x: R|kotlin/String|by R|/lazy|<R|kotlin/String|>(<L> = lazy@fun <anonymous>(): R|kotlin/String| {
-            lval y: R|kotlin/String| = when (lval <elvis>: R|kotlin/String?| = (R|/getAny|() as? R|kotlin/String|)) {
-                ==($subj$, Null(null)) ->  {
-                    String()
-                }
-                else ->  {
-                    R|<local>/<elvis>|
-                }
-            }
-
+            lval y: R|kotlin/String| = (R|/getAny|() as? R|kotlin/String|) ?: String()
             ^ R|<local>/y|
         }
         )