Fix default display in help for EnumerableFlag and other types

[natecook1000]

When an EnumerableFlag flag has a default value, the default is printed incorrectly as the instance value in the help, instead of as the flag that a user would need to type. The same issue is present for Boolean flags with both an inversion and a default.

struct Example: ParsableCommand {
   @Flag(inversion: .prefixedNo)
   var frobulate: Bool = true
}

Old help output:

USAGE: example [--frobulate] [--no-frobulate]

OPTIONS:
  --frobulate/--no-frobulate
                          Whether to frobulate. (default: true)
  -h, --help              Show help information.

Corrected help output:

USAGE: example [--frobulate] [--no-frobulate]

OPTIONS:
  --frobulate/--no-frobulate
                          Whether to frobulate. (default: --frobulate)
  -h, --help              Show help information.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rauhul]

Ooh this is a nice fix!

[natecook1000]

Will almost certainly conflict with #477, let's land that first!

[natecook1000]

@swift-ci Please test