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

[Tom Honermann via llvm-commits]

https://github.com/tahonermann approved this pull request.

Thank you for all your work on this @KseniyaTikhomirova and for tolerating all of my questions and requests! This looks great!

There is one remaining comment that is unresolved that I just responded to. I'm approving this PR based on my expectation that the code expresses the intent as is. However, if the intent really is that debug builds of libsycl are always required to link with a debug version of the MSVC CRT, then I think an additional change is warranted and we should follow up in that comment thread. But I suspect that isn't actually the intent and we are good here.

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