choice: don't require empty `()`

[jonmeow]

https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/README.md#choice-types has example code:

choice IntResult {
  Success(value: i32),
  Failure(error: String),
  Cancelled
}

...

  case .Cancelled => {
    Terminate();
  }

Right now, it would be Cancelled() with parentheses: https://carbon.compiler-explorer.com/z/3dnK4xP38

Probably the right choice here is to not require parentheses in the explorer, although that'll probably require some extra handling of choices so that the parentheses are only conditionally required.

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[chandlerc]

Closing explorer-specific issues as not-planned for now due to our decision to prioritize working on the toolchain over other implementation work in the near term: https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3532.md