[BUG] Inheritance is broken

Created by: jonrimmer

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?

Description

Basic inheritance, as described in the OpenAPI spec does not work.

Instead of creating a class / interface hierarchy using inheritance to represent the composition, all properties are flattened into the subclass.

openapi-generator version

4.0.0-20190508.072036-627 or master

OpenAPI declaration file content or url

schema.yaml:

openapi: "3.0.1"
info:
  version: 1.0.0
  title: Inheritance test
paths:
  /order:
    get:
      summary: Product order details
      operationId: getOrder
      responses:
        '500':
          description: Successful load
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ExtendedErrorModel'
components:
  schemas:
    BasicErrorModel:
      type: object
      required:
        - message
        - code
      properties:
        message:
          type: string
        code:
          type: integer
          minimum: 100
          maximum: 600
    ExtendedErrorModel:
      allOf:
        - $ref: '#/components/schemas/BasicErrorModel'
        - type: object
          required:
            - rootCause
          properties:
            rootCause:
              type: string

Expected output

The following class/interface hierarchy: to be generated:

BasicErrorModel <- ExtendedErrorModel

Actual output

Three unrelated classes/interfaces:

BasicErrorModel
ExtendedErrorModel
ExtendedErrorModelAllOf

Command line used for generation

java -jar openapi-generator-cli.jar generate -i ./schema.yaml -g typescript-angular -o src

I have also tried with the Java generator, and it creates the same hierarchy, so it is not just a Typescript problem.

Steps to reproduce

Copy the above yaml into schema.yaml locally.
Run the command above.
Examine the generated code.

Suggest a fix

I'm not familiar with the codebase, but it appears that getParentName in ModelUtils.java only considers a class as a viable parent for inheritance if it has a discriminator, but the above inheritance pattern does not use discriminators.