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

[Kseniya Tikhomirova via llvm-commits]

KseniyaTikhomirova wrote:

> Also, it would be great if we could get the precommit CI running ASAP. IIUC the basic configurations would be:
> 
> * libstdc++/GCC(?)
> * MSVC STL/MSVC
> * libunwind/libc++abi/libc++/Clang
>   That should be pretty lean on the CI resources and could probably easily run in the GH-provided runners for now.

Could you please clarify - is it a blocker for the current PR? Can we add it in the following PRs?

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