[BUG] Fix parentVars and emptyVars in Swift 5 generator

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output? - In case of ClassC > ClassB > ClassA, ClassC's parentVars doesn't contain properties of ClassB, only of ClassA.
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating Swift 5, the parentVars property was wrongly set for multi-level inheritance (Class C > Class B > Class A). Also emptyVars was never set.

openapi-generator version

5.1.1

OpenAPI declaration file content or url

{
  "swagger": "2.0",
  "info": {
    "version": "1.0.5",
    "title": "Swagger Petstore"
  },
  "paths": {

  },
  "definitions": {
    "ClassA": {
      "discriminator": "@type",
      "type": "object",
      "properties": {
        "propertyA": {
          "type": "string"
        }
      }
    },
    "ClassB": {
      "allOf": [
        {
          "$ref": "#/definitions/ClassA"
        },
        {
          "type": "object",
          "discriminator": "@type",
          "properties": {
            "propertyB": {
              "type": "string"
            },
            "propertyA": {
              "type": "string"        
            }
          }
        }
      ]
    },
    "ClassC": {
      "allOf": [
        {
          "$ref": "#/definitions/ClassB"
        },
        {
          "type": "object",
          "properties": {
            "propertyA": {
              "type": "string"
            },
            "propertyB": {
              "type": "string"
            },
            "propertyC": {
              "type": "string"
            }
          }
        }
      ]
    }
  }
}

Generation Details

When running OAG with debugModels flag, the mentioned values are wrongly shown, because they're wrongly set.

Steps to reproduce

Run OAG with the above posted JSON as input and debugModels flag enabled.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/9244

Suggest a fix

Fixed in https://github.com/OpenAPITools/openapi-generator/pull/9244

Start by running OpenAPI Generator with the supplied JSON input, the Swift 5 generator, and the debugModels flag. Inspect the Swift 5 generator's handling of multi-level inheritance; done means Class C's parentVars includes Class B properties and emptyVars is populated, with regression coverage for the example.