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

[Alexey Sachkov via llvm-commits]

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AlexeySachkov wrote:

+1 here. We already have people using "internal" `sycl/` headers whose names, contents and mere existence are not guaranteed by any specifications.

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