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

[Kseniya Tikhomirova via llvm-commits]

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KseniyaTikhomirova wrote:

Ok, I can rename it.

@AlexeySachkov I believe they do it intentionally. Do you think `__detail` will help to avoid that?

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