[Swift4] Primitive typealias produces void class

Description

We have a handful of primitive "typealias" definitions in yaml, like so

TokenId:
  type: string

which in codegen 2.2 produces
typealias TokenId = String

but in codegen 2.3.3 and 3.0.0 produces

open class TokenId: Codable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: String.self)
    }
    public required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: String.self)
    }
}

which is a "void" implementation, as it does not declare any usable properties.

Swagger-codegen version

2.3.3, 3.0.0

Swagger declaration file content or url

paths is not in focus of this issue and may be empty per OpenAPI spec, but codegen throws NPE without it.

swagger: "2.0"
info:
  title: "Test"
  version: "1.0"
paths:
  '/':
    get:
      responses:
        '200':
          schema:
            $ref: '#/definitions/TokenId'
definitions:
  TokenId:
    type: string

Command line used for generation

java -jar swagger-codegen-cli-x.y.z.jar generate -i swagger.yaml -l swift4 -o ./codegen

Steps to reproduce

Run codegen with the above yaml

Related issues/PRs

Possibly, vaguely, in other language(s)
https://github.com/swagger-api/swagger-codegen/issues/3483
https://github.com/swagger-api/swagger-codegen/issues/4804
but mind that this was working up to 2.2, just not since 2.3 with introduction of Codable.

Suggest a fix/enhancement

As Swift primitives are Codables right away, use typealias as in 2.2 ?

Start with the Swift4 generator path used by the java -jar swagger-codegen-cli-x.y.z.jar generate command and the handling of primitive definitions such as TokenId in the supplied Swagger YAML. Compare generated output with the expected typealias TokenId = String, and use the reproduction command to verify that primitive aliases no longer become empty Codable classes.