[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.

This is looking good @schittir! I added some comments for minor issues. I still need to review the tests more closely.

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