Don't let .allUnrecognized consume built-in flags

[natecook1000]

Arguments declared with the .allUnrecognized parsing strategy currently capture built-in flags, which isn't intended. This fixes that issue by looking for built-in flags in the captured portion of the input before decoding.

Fixes rdar://104990388

Checklist

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

[natecook1000]

@swift-ci Please test