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

[Tom Honermann via cfe-commits]

tahonermann wrote:

I missed the previous PRs that lumped all of these attributes together. I think `sycl_kernel` should have been left separate as it isn't directly used to declare a device function. It instead has rather complicated semantics that result in a function that is used on the host as well as a synthesized device function (with a different name, different parameters, and a hard-coded calling convention). However, `sycl_kernel` is also destined to go away in favor of the `sycl_kernel_entry_point` attribute.

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