[clang] [clang] Fix inconsistencies with the device_kernel attr on different targets (PR #161905)

[Tom Honermann via cfe-commits]

tahonermann wrote:

> Joseph's suggestion seems the best to me, having one of the aliases have totally different semantics is just asking for trouble. If @tahonermann agrees I'll update this PR to implement that.

Yes, please. Enablement of the `sycl_kernel` attribute is already subject to use of the `-fsycl` option. Please do revert the prior change to the documentation and add appropriate documentation for `device_kernel` and friends. I suggest doing this in a separate PR (probably one that lands before this one).

https://github.com/llvm/llvm-project/pull/161905