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

[Kseniya Tikhomirova via llvm-commits]

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KseniyaTikhomirova wrote:

I renamed files which are added in this PR.
install results looks like:
-- Installing: <some dir>/install/release/include/sycl/__version.hpp
-- Installing: <some dir>/install/release/include/sycl/sycl.hpp
-- Installing: <some dir>/install/release/include/sycl/__detail
-- Installing: <some dir>/install/release/include/sycl/__detail/__config.hpp
-- Installing: <some dir>/install/release/include/sycl/__platform.hpp
-- Installing: <some dir>/install/release/lib/libsycl.so.0.1.0
-- Installing: <some dir>/install/release/lib/libsycl.so

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