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

[via cfe-commits]

schittir wrote:

> This is looking great @schittir! I just realized that, since we made the change to allow the attribute to be present during host compilation that we should no longer restrict diagnostics to device compilation and that tests should exercise host compilation as well.
> 
> I suggested a few FIXME comments, but other than that, I think this is good to go!

Thank you for the review, Tom! 

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