`usage` is not correctly calculated for the enum when the body is an enum

For the following typespec:

    @put
    @route("/unknown-value")
    @doc("get extensible enum")
    op createUnknownValue(@body input: DaysOfWeekExtensibleEnum): void;

union DaysOfWeekExtensibleEnum {
    string,
    Monday: "Monday",
    Tuesday: "Tuesday",
    Wednesday: "Wednesday",
    Thursday: "Thursday",
    Friday: "Friday",
    Saturday: "Saturday",
    Sunday: "Sunday",
}

The usage of DaysOfWeekExtensibleEnum should be input, but TCGC gives us None.

Azure / typespec-azure

`usage` is not correctly calculated for the enum when the body is an enum #1051