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

[Kseniya Tikhomirova via llvm-commits]

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KseniyaTikhomirova wrote:

All our includes are done and expected to be done via "**sycl**/...", "**sycl**/**detail**/...". So I expect it to be pretty specific to avoid name clashes with user headers. What do you think?


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