[SYCL][NFC] Don't use std::ignore

[AlexeySachkov]

Its implementation involve templates, which means slow compile times. [[maybe_unused]] does the job just fine, but it is cheaper.

This patch only changed sycl/ headers, ignoring sycl/ext headers.

We still maintain the fact that our SYCL RT library should be buildable with an old GCC (7.5.0 to be precise) and it doesn't support [[maybe_unused]] C++17 attribute. To avoid the attribute "leaking" into the library code, some of the image accessor code has been split into multiple headers to reduce the surface of what is being included from the library sources.

[AlexeySachkov]

This PR prompted by the following failure, observed here in pre-commit for #19936. Whilst I believe the failure to be some kind of merge conflict error, it is a good opportunity to stop using std::ignore anyways.

Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 
FAIL: SYCL :: self-contained-headers/sycl/group_barrier.hpp (641 of 903)
******************** TEST 'SYCL :: self-contained-headers/sycl/group_barrier.hpp' FAILED ********************
Running command: /__w/llvm/llvm/build/bin/clang -fsycl -fsyntax-only -include /__w/llvm/llvm/build/include/sycl/group_barrier.hpp /__w/llvm/llvm/src/sycl/test/self-contained-headers/Inputs/test.cpp
Exit Code: 1

Command Output (stderr):
--
In file included from <built-in>:1:
In file included from /__w/llvm/llvm/build/include/sycl/group_barrier.hpp:12:
/__w/llvm/llvm/build/bin/../include/sycl/detail/spirv.hpp:927:8: error: no member named 'ignore' in namespace 'std'
  927 |   std::ignore = g;
      |        ^~~~~~
/__w/llvm/llvm/build/bin/../include/sycl/detail/spirv.hpp:960:8: error: no member named 'ignore' in namespace 'std'
  960 |   std::ignore = g;
      |        ^~~~~~
2 errors generated.

[AlexeySachkov]

At this point, it can probably be closed in favor of #20088, even though I personally like the attribute better than the void cast. Tagging @aelovikov-intel for awareness

[aelovikov-intel]

Whilst I believe the failure to be some kind of merge conflict error, it is a good opportunity to stop using std::ignore anyways.

It's probably specific to the default gcc toolchain used in Ubuntu 22.04 (that is the failing task in your pre-commit, which is essentially Nightly workload where the issue had happened too).