The @discriminator tag does not handle `ref` type discriminators

[PLkolek]

Very similar (hence I copied the description and just substituted inputs and outputs) issue to https://github.com/vega/ts-json-schema-generator/issues/1682, but now caused by using an exported type as a discriminator.

For example,

export enum AEnum {
    a = "a",
    A = "A"
}

type A = { kind: AEnum, a: string }
type B = { kind: "b" | "B", b: string }

/**
 * @discriminator kind
 */
export type AB = A | B;

results in the following schema for the type AB:

Schema result

{
  "$id": "api.json",
  "$schema": "http://json-schema.org/draft-07/schema#",
  "definitions": {
    "AB": {
      "allOf": [
        {
          "if": {
            "properties": {
              "kind": {
                "$ref": "#/definitions/AEnum"
              }
            }
          },
          "then": {
            "additionalProperties": false,
            "properties": {
              "a": {
                "type": "string"
              },
              "kind": {
                "$ref": "#/definitions/AEnum"
              }
            },
            "required": [
              "kind",
              "a"
            ],
            "type": "object"
          }
        },
        {
          "if": {
            "properties": {
              "kind": {
                "enum": [
                  "b",
                  "B"
                ],
                "type": "string"
              }
            }
          },
          "then": {
            "additionalProperties": false,
            "properties": {
              "b": {
                "type": "string"
              },
              "kind": {
                "enum": [
                  "b",
                  "B"
                ],
                "type": "string"
              }
            },
            "required": [
              "kind",
              "b"
            ],
            "type": "object"
          }
        }
      ],
      "properties": {
        "kind": {
          "enum": [
            "b",
            "B"
          ]
        }
      },
      "required": [
        "kind"
      ],
      "type": "object"
    },
    "AEnum": {
      "enum": [
        "a",
        "A"
      ],
      "type": "string"
    }
  }
}

Where the set of accepted values for the outermost kind enum is does not contain the values originating from AEnum, whereas it should list all possible options.