[clang][Sema] Split SYCLKernel back into its own attribute (#162868)
Based on feedback from https://github.com/llvm/llvm-project/pull/161905,
partially revert https://github.com/llvm/llvm-project/pull/137882/ so
that `sycl_kernel` is a separate attribute and not just a spelling of
`device_kernel`.
Signed-off-by: Sarnie, Nick <nick.sarnie@intel.com>