[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/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -1022,6 +1022,12 @@ class FirRenderer(builder: StringBuilder, private val mode: RenderMode = RenderM
         print("!!")
     }
 
+    override fun visitElvisCall(elvisCall: FirElvisCall) {
+        elvisCall.lhs.accept(this)
+        print(" ?: ")
+        elvisCall.rhs.accept(this)
+    }
+
     override fun visitCallableReferenceAccess(callableReferenceAccess: FirCallableReferenceAccess) {
         callableReferenceAccess.annotations.renderAnnotations()
         callableReferenceAccess.explicitReceiver?.accept(this)