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

[Tom Honermann via cfe-commits]

https://github.com/tahonermann approved this pull request.

I love it @schittir, this looks great!

@erichkeane, @bader, please review and lend your approval. Note that some of the changes to the tests are to synchronize them with continued evolution that happened in DPC++ but was not previously upstreamed.

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