[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann requested changes to this pull request.

Thanks for addressing most of my earlier comments. I took another quick look today and added a couple more comments.

https://github.com/llvm/llvm-project/pull/140282