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

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elizabethandrews wrote:

> Thanks @elizabethandrews, I think this looks great! I noted one place where I'm not sure if a change that was made is still needed. I'll follow up with documentation suggestions as we discussed (probably not today though).

Thanks for the review @tahonermann. Will the current documentation suffice for this PR? If so, I could open this up for review and you could do a follow-up PR with more documentation. 

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