[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/invokeInWhenSubjectVariableInitializer.txt

@@ -1,12 +1,4 @@
 FILE: invokeInWhenSubjectVariableInitializer.kt
     public final fun test(func: R|() -> kotlin/String?|): R|kotlin/Unit| {
-        lval x: R|kotlin/String| = when (lval <elvis>: R|kotlin/String?| = R|<local>/func|.R|FakeOverride<kotlin/Function0.invoke: R|kotlin/String?|>|()) {
-            ==($subj$, Null(null)) ->  {
-                String()
-            }
-            else ->  {
-                R|<local>/<elvis>|
-            }
-        }
-
+        lval x: R|kotlin/String| = R|<local>/func|.R|FakeOverride<kotlin/Function0.invoke: R|kotlin/String?|>|() ?: String()
     }