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

[Nikolas Klauser 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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philnik777 wrote:

"TODO" is used a lot more in llvm than "TBD", so people are less likely to grep for it.

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