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

[Kseniya Tikhomirova via llvm-commits]

KseniyaTikhomirova wrote:

> I think it would be useful to also add the `include/CL` directory and the (deprecated) `include/CL/sycl.hpp` header file to fully flush out the header file organization. https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:headers-and-namespaces.

done in https://github.com/llvm/llvm-project/pull/144372/commits/033d151f2a9ae6c6e847e73c5cf9f1bbe482ce21 (added file) and https://github.com/llvm/llvm-project/pull/144372/commits/d4a2dc494f6744963ebb94e041d57c3131b49ca0 (cmake file update)

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