Handle subcommands

[sjavora]

This tries to implement the handling of subcommands based on the original driver. I must admit I'm a little out of my depth here so I'd appreciate some guidance:

• in the original driver, the subcommand check happens before the actual driver is even created (here) - I put the code in the init, because the FIXME was there, but should an init be invoking exec?
• related to the above, is exec the right function to use here?
• similarly, is Process.findExecutable the right method to use in place of getExecutablePath?
• there is a slight difference in behavior between llvm::sys::path::stem and basenameWithoutExt - namely basenameWithoutExt will return .txt for /foo/.txt, while llvm::sys::path::stem will return an empty string. Is this a problem? If so, should I implement something like stem on VirtualPath?

Thank you very much for your time 🙂

[DougGregor]

I think we should throw here, so we don't continue trying to parse the rest of the command line.

[sjavora]

Sure thing, but then we can't check the exact error type in the tests, since Driver.Error is internal - should it be made public?

Alternatively, we could add descriptions to the errors and match those, but I tried overriding implementing localizedDescription and it was not called for some reason.

[harlanhaskins]

LGTM!

[harlanhaskins]

@swift-ci please test

[sjavora]

@DougGregor is there anything else I should change here?

[DougGregor]

Looks great, thank you!