[llvm] [SYCL] Add libsycl, a SYCL RT library implementation project (PR #144372)

[Tom Honermann via llvm-commits]

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

Thanks @KseniyaTikhomirova, this is looking great. I added a few final comments, but will be very happy to approve once we address these.

https://github.com/llvm/llvm-project/pull/144372