[clang] [clang][SYCL] Add SYCL spelling for AS attributes (PR #200849)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann requested changes to this pull request.

Thanks, @elizabethandrews! I added a number of minor comments, but I think this is looking good. Some of the suggestions I added might turn out to be disruptive or just plain bad ideas in which case we should discuss and ignore those or perhaps address them in a later PR.

https://github.com/llvm/llvm-project/pull/200849