[Bug]: Loss the xml entry for enum, union

### Describe the bug

[Playground](https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMveG1sIjsKCnVzaW5nIFhtbDsKCm1vZGVsIEJvb2sgewogIHN0YXR1czogRW51bVPFDDsKICB1bmlvbnM6VcQHRGF0YVtdCn0KCkBuYW1lKCJYbWzKLiIpCmVudW3LQCB7IAogIEFjdGl2ZSwKICBJbmHFDDoiyAoiLM5OyWciKQrFeiDJEuUApGE6IsZPIsRQYstJCn0K&e=%40typespec%2Fopenapi3&options=%7B%7D)

the `EnumStatus` and `UnionData` has no xml property.

### Reproduction
```yaml
openapi: 3.0.0
info:
title: (title)
version: 0.0.0
tags: []
paths: {}
components:
schemas:
Book:
type: object
required:
- status
- unions
properties:
status:
allOf:
- $ref: '#/components/schemas/EnumStatus'
xml:
name: status
unions:
type: array
items:
allOf:
- $ref: '#/components/schemas/UnionData'
xml:
name: XmlUnionData # ---> not sure if expect.
xml:
wrapped: true
EnumStatus:
type: string
enum:
- Active
- Inactive
xml:
name: XmlEnumStatus # ---> expected
UnionData:
type: string
enum:
- Active
- Inactive
xml:
name: XmlUnionData # ---> expected
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.