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

[Nikolas Klauser via llvm-commits]

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philnik777 wrote:

I'm not a huge fan of the placing of the header, but if it makes the CMake significantly easier I don't think it's worth changing. I guess I'd like some way to say "except *.in files", but we don't have that.

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