[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 @schittir, here are my initial review comments. I expect to review the newly added tests more closely once these comments are all addressed.

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