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

[Kseniya Tikhomirova via llvm-commits]

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+## SYCL runtime library
+
+The libsycl subproject is an implementation of the SYCL runtime library as defined by the
+[SYCL 2020 specification](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html).
+
+Subproject documentation is available at: [SYCL RT documentation](./docs).
+
+libsycl runtime library and headers require C++17 support or higher.
+
+### How to use libsycl with Clang
+
+TBD
+
+#### How to build
+
+TBD
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KseniyaTikhomirova wrote:

Yes, this is planned to be filled in future PRs, added some sections to show that the project will be properly documented. 
I can replace "TBD" with "TODO", of course, although I don't see any difference in grepping by "TBD" or "TODO".
Please let me know your thoughts. Thanks.

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