[SYCL] Require emit of sycl_kernel_entry_point attributed functions.

[tahonermann]

Initial device-side code generation support for functions declared with the sycl_kernel_entry_point was recently added to upstream Clang, but did not take effect due to downstream customizations. This change adjusts the customizations and updates the associated test for other downstream customizations.

[jsji]

@intel/dpcpp-cfe-reviewers Please review. Thanks.

[tahonermann]

Closing this PR as Jinsong committed them long ago. Concerns raised by Mariya are partially addressed by https://github.com/llvm/llvm-project/pull/140282; the remaining ones will be addressed by https://github.com/intel/llvm/issues/18304.