[clang] [SYCL] AST support for SYCL kernel entry point functions. (PR #122379)

[Alexey Bader via cfe-commits]

https://github.com/bader approved this pull request.

@tahonermann thanks a lot for the work. I really appreciate the detailed description and comments!

@erichkeane did a great job with the code review, so I have nothing to add.

I just have one small question that does not affect the merge.

https://github.com/llvm/llvm-project/pull/122379