ExpressibleByArgument, CaseIterable for enum not listing options during failure

[BadPirate]

When using ExpressibleByArgument for an enum @option, neither help nor error message correctly shows available options.

ArgumentParser: main
Swift version: Apple Swift version 5.3.1

Checklist

• If possible, I've reproduced the issue using the main branch of this package
• I've searched for existing GitHub issues

Steps to Reproduce

Create an option:

    enum Config : String, ExpressibleByArgument, CaseIterable
    {
        case ios, mac, simulator
    }
    
    @Option(help: "Build configuration")
    var config : [Config] = [.ios]

See command help screen using -h, or use the config option with an illegal value:

cli command --config invalid

Expected behavior

1. Help should show options available
2. Error message should show valid options

Actual behavior

1. Help Provides no options.
2. Error provides no useful options:

Error: The value 'invalid' is invalid for '--config <config>'
Usage: cli command [--config <config> ...] 
  See 'cli command --help' for more information.

[natecook1000]

Resolved by #382.