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

[Kseniya Tikhomirova via llvm-commits]

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KseniyaTikhomirova wrote:

done https://github.com/llvm/llvm-project/commit/e901d3015eeb76e7216dba6f7383590b469af315
although we don't have "overview" section there. 

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