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

[Nikolas Klauser via llvm-commits]

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+BasedOnStyle: LLVM
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philnik777 wrote:

We set it in libc++ becuase it makes things a lot more readable in `__config`. compiler-rt sets it as well.

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