[clang] [clang][SYCL] Implement address space attributes for SYCL (PR #200849)

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elizabethandrews wrote:

> I'm not sure that top-level type of a variable declaration is actually any better than the 'must be object pointer or reference'? Maybe 'must be pointer or reference', and let the function pointer check take care of clarifying it?

Do you mean `"'%0' attribute may only be applied to a pointer or reference"`

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