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

[Tom Honermann via llvm-commits]

https://github.com/tahonermann requested changes to this pull request.

Thank you for responding to my prior comments. I added a number of new comments, many of which are minor. I would really like to see some of the differences between windows and Linux/UNIX build differences eliminated; particularly the library naming differences and the "d" debug postfix naming. The default build should produce binaries named appropriately for a LLVM/Clang monorepo build and distribution (which shouldn't differ much between Windows and Linux/UNIX).

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