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

[Kseniya Tikhomirova via llvm-commits]

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KseniyaTikhomirova wrote:

Well, I think that we use this approach to store file that is just an input for generator because of the way we copy headers. Probably a good chance to discuss this approach too. 
I noticed that other projects do copy per file instead of doing copy of whole `include` directory.
I think libcxx uses file list for headers to use copy_if_different tool that copies files only if they were changed. This approach allows to store input files in the same folder with full header files.
copy_directory_if_different is added in later version, in 3.26.
We use copy_directory that doesn't detect changes but works faster. At early stages I believe there is no noticeable difference.
Do you know if there is any other reason for other projects to copy headers per file?

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