[BUG][C++][QT5] Code not compileable when circular scheme dependencies exist

Created by: ghost

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

The generated C++ (QT5) code cannot be compiled, when circular scheme dependencies are used.

openapi-generator version

4.3.1

OpenAPI declaration file content or url

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'

Command line used for generation

java -cp openapi-generator-cli-4.3.1.jar org.openapitools.codegen.OpenAPIGenerator generate -i ${SPEC_FILE} -g cpp-qt5-client -o gen/ --model-name-prefix Test

Steps to reproduce

1. Generate CPP code with above command line.
2. Generate Makefiles e.g. with (MAC and QT5.14.0): cmake CMakeLists.txt -DCMAKE_PREFIX_PATH=/usr/local/Cellar/qt/5.14.0/
3. Compile: make

Related issues/PRs

Few similar issues (other languages) seems to exist.

Suggest a fix

Partly it can probably be fixed by forward declaration in headers, but not completely. More investigation needed.