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

[Kseniya Tikhomirova via llvm-commits]

KseniyaTikhomirova wrote:

now install files look like:
```
-- Up-to-date: <some dir>/install/release/include/sycl/__detail/version.hpp
-- Up-to-date: <some dir>/install/release/include/sycl
-- Up-to-date: <some dir>/install/release/include/sycl/sycl.hpp
-- Up-to-date: <some dir>/install/release/include/sycl/__detail
-- Up-to-date: <some dir>/install/release/include/sycl/__detail/impl
-- Up-to-date: <some dir>/install/release/include/sycl/__detail/impl/config.hpp
-- Up-to-date: <some dir>/install/release/include/sycl/__detail/platform.hpp
-- Up-to-date: <some dir>/install/release/include/CL/sycl.hpp
-- Up-to-date: <some dir>/install/release/lib/libsycl.so.0.1.0
-- Up-to-date: <some dir>/install/release/lib/libsycl.so
```

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