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

[Nikolas Klauser via llvm-commits]

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philnik777 wrote:

I think it would be better to name the directory `__detail`. That avoids any name clashes with user headers.

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