Use swift-argument-parser in generate-swift-syntax-builder

[fwcd]

This branch updates generate-swift-syntax-builder to use swift-argument-parser instead of ad-hoc parsing CommandLine.arguments.

Since the package already depends on swift-argument-parser for swift-parser-test I would not expect the introduction of this new target dependency to be an issue, but perhaps still worth checking whether there are any unexpected bootstrapping consequences to this.

[fwcd]

@swift-ci please test

[ahoppen]

I agree that there should not be any issues associated with using swift-argument-parser for generate-swift-syntax-builder. The important bit is that we would still be able to build SwiftParser without depending on swift-argument parser.

[fwcd]

@swift-ci please test