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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

If the `sycl_kernel` version has all this special behavior it should be a separate attribute and put under the SYCL language mode. The others should all just be aliases to `device_kernel` and have those do some unified behavior, imply non-hidden visibility (if default), and add the appropriate target calling convention (if one exists).

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