jextract: Swift 6 language compatibility for generated sources

[madsodgaard]

Right now, the generated code will fail to compile in the Swift 6 language mode. We should fix this, so developers don't have to revert back to Swift 5.

[ktoso]

Yes, though it might be more helpful to be specific with this issue.

I think we likely have the same issue as this one but for jextract 🤔 // edited

• [Swift-6] Get rid of type shadowing warnings in generated sources #169 - this is quite annoying to solve but doable, and we'd welcome contributions for it!

[ktoso]

Meh, I confused modes. Pretty sure we have the same issue in jextract though 🤔