[C++]Enable forward declaration for C++

Created by: etherealjoy

Fixes #6377 (closed) @muttleyxd @MartinDelille @stkrwork @ravinikam @wing328

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Fixes this spec below

openapi: 3.0.0
info:
  version: 0.0.1
  title: Test
paths:
  /endpoint:
    get:
      operationId: getOptions
      description: "-"
      parameters:
      - name: OptionA
        in: header
        schema:
          $ref: '#/components/schemas/OptionA'
      responses:
        200:
          description: Success
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/OptionA'
components:
  schemas:
    OptionA:
      type: object
      description: 'OptionA'
      properties:
        suboptions:
          type: array
          description: 'Suboptions of OptionA'
          items:
            $ref: '#/components/schemas/OptionB'
    OptionB:
      type: object
      description: 'OptionB'
      properties:
        suboptions1:
          type: array
          description: 'Suboptions 1 of OptionB'
          items:
            $ref: '#/components/schemas/OptionA'
        suboptions2:
          type: array
          description: 'Suboptions 2 of OptionB'
          items:
            $ref: '#/components/schemas/OptionB'

This spec below cannot be fixed due to C++ limitations.

openapi: 3.0.0
info:
  version: 0.0.1
  title: Test
paths:
  /endpoint:
    get:
      operationId: getOptions
      description: "-"
      parameters:
      - name: OptionA
        in: header
        schema:
          $ref: '#/components/schemas/OptionA'
      responses:
        200:
          description: Success
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/OptionA'
components:
  schemas:
    OptionA:
      type: object
      description: 'OptionA'
      properties:
        suboptions:
          $ref: '#/components/schemas/OptionB'
    OptionB:
      type: object
      description: 'OptionB'
      properties:
        suboptions1:
          $ref: '#/components/schemas/OptionA'

Generated code below has forward declaration and include as well.

#ifndef OAIOptionB_H
#define OAIOptionB_H

#include <QJsonObject>

#include "OAIOptionA.h"

#include "OAIEnum.h"
#include "OAIObject.h"

namespace OpenAPI {
class OAIOptionA;

class OAIOptionB : public OAIObject {
public:
    OAIOptionB();
    OAIOptionB(QString json);
    ~OAIOptionB() override;

    QString asJson() const override;
    QJsonObject asJsonObject() const override;
    void fromJsonObject(QJsonObject json) override;
    void fromJson(QString jsonString) override;

    OAIOptionA getSuboptions1() const;
    void setSuboptions1(const OAIOptionA &suboptions1);
    bool is_suboptions1_Set() const;
    bool is_suboptions1_Valid() const;

    virtual bool isSet() const override;
    virtual bool isValid() const override;

private:
    void initializeModel();

    OAIOptionA suboptions1;
    bool m_suboptions1_isSet;
    bool m_suboptions1_isValid;
};

} // namespace OpenAPI

Error for the one above

OAIOptionB.h:53: error: field ‘suboptions1’ has incomplete type ‘OpenAPI::OAIOptionA’
   53 |     OAIOptionA suboptions1;
      |                ^~~~~~~~~~~