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

[Kseniya Tikhomirova via llvm-commits]

KseniyaTikhomirova wrote:

> I don't think it is necessary for files present in the `__detail` directory to be prefixed with an `__`; their presence in that directory already signifies that they are not public header files. The `include/sycl/__detail/__config.hpp` file therefore doesn't need the `__` prefix. `__platform.hpp` should still be named with a `__` prefix though since it is not a public header file and is not located in a `__` prefixed directory. This is consistent with what libc++ does; see https://github.com/llvm/llvm-project/tree/main/libcxx/include.
> 
> The number of header files that are prefixed with an `__` can be reduced by placing all non-public header files in some `__` prefixed directory; perhaps the `__detail` directory or split in some fashion. For example, a header file like `platform.hpp` that implements a public facing interface specified in the SYCL specification could be placed in a `__implementation` or `__impl` directory with the `__detail` directory reserved for things like the DPC++ program manager that really implementation detail. I don't have strong opinions how the files are structured so long as non-public header files are either in a `__` prefixed directory or themselves have a `__` prefix (but not both).

updated https://github.com/llvm/llvm-project/pull/144372/commits/d4a2dc494f6744963ebb94e041d57c3131b49ca0

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