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

[Nikolas Klauser via llvm-commits]

philnik777 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.

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