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

[Kseniya Tikhomirova via llvm-commits]

https://github.com/KseniyaTikhomirova commented:

[philnik777](https://github.com/philnik777) thank you for the review, I really appreciate your comments and guidance.

I replied to all your comments and will provide code updates a bit later.

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