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

[Nikolas Klauser via llvm-commits]

================
@@ -0,0 +1,20 @@
+## 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
----------------
philnik777 wrote:

I guess this will be filled in future PRs? Maybe Make this a TODO instead? That's usually easier to grep for.

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