[SYCL] Remove sycl_ext_oneapi_sub_group in preview-breaking mode

[softwaresat]

Guards the deprecated SYCL_INTEL_sub_group feature-test macro from preview-breaking builds.

Documentation is marked for removal or relocation during the next breaking-changes window.

This closes #22581.

[KornevNikita]

This descriptor and all related code should be removed:

llvm/sycl/include/sycl/info/device.hpp

Line 466 in 0f637d2

sub_group_independent_forward_progress

This header should be removed:
https://github.com/intel/llvm/blob/sycl/sycl/include/sycl/ext/oneapi/sub_group.hpp

I guess the rest of code that should be removed will be easy to find if you disable this header and try to compile the project.

[gmlueck]

This deprecated extension also defines a couple of C++ attributes:

• [[intel::sub_group_size(S)]]
• [[intel::named_sub_group_size(NAME)]]

These are implemented in the C++ front-end (not the headers). I think we should deprecate these also. I don't know if there is a way to remove them from the CFE in -fpreview-breaking-changes. If you want to assign this part of the task to the CFE team, that's fine. But, we should create a tracker in that case.

Note that we have functionality similar to [[intel::named_sub_group_size(NAME)]] proposed in sycl_ext_oneapi_named_sub_group_sizes, but that proposal uses properties (not attributes). Therefore, even if we implement that extension, I don't think we would want that attribute.