[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

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

Thanks for working on this @schittir! I completed an initial pass of all of the code, but still need to look more closely at the documentation updates.

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