Convert Generator to swift-argument-parser

[pennig]

This PR migrates the command line parsing code provided by swift-tools-support-core, which has long been replaced by the standalone swift-argument-parser package. It eliminates all the deprecation warnings, and cleans up the generator command code and file structure.

I also opted to update swift-tools-version to 5.2, since I presumed that since Swift 5.1 code is no longer supported by the generator itself, that anybody compiling the generator would not require the 5.1 toolset.

[CLAassistant]

All committers have signed the CLA.

[wbond]

It looks like CI isn't happy here. I haven't had the time to debug yet, but I am keeping this on my list of things to devote some time to.

Sorry for the delay!

[wbond]

I tried to update the GitHub Actions CI config to a newer version of Xcode, but it doesn't seem to be running.

Do you mind trying to rebase on master and pushing up to see if CI runs then? I think you'll need to add a new parameter added in a recent commit.

[pennig]

@wbond added that parameter. I must not have gotten the notification or email from your comment