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

[Tom Honermann via llvm-commits]

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

Thank you for all your work on this @KseniyaTikhomirova! I'm happy with this and therefore approving. I leave it to you If you want to do anything to close out [this comment](https://github.com/llvm/llvm-project/pull/144372#discussion_r2205750367). 

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