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

[Alexey Bader via llvm-commits]

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bader wrote:

Intel's implementation uses `detail` because we wanted the directory hierarchy and names are matching namespace hierarchy and names from the SYCL specification.
See https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:headers-and-namespaces.

I don't have strong opinion, but if we assume that `<sycl/detail>` might conflict with user's headers, we will have to and underscore prefixes too all directories and headers as SYCL spec defines only one header `<sycl/sycl.hpp>`.

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