Add infrastructure for working with concepts

[stephenswat]

This commit adds the TRACCC_CONSTRAINT macro which acts as a concept in C++20 builds, and simply acts as the typename keyword in builds with older standards. It also adds the TRACCC_ENFORCE_CONCEPTS CMake flag which will throw an error if concepts are not available.

[stephenswat]

Okay it took some faffing about because of CMake but this is ready to go.

[krasznaa]

I do not like the change to ci_setup.sh. It should not be in the business of making decisions about compiler flags like that. 😦

I remember that you found that older CMake versions don't respect CMAKE_CUDA_STANDARD as they should. But then, let's do something similar to what we settled on in acts-project/acts#3318 with @paulgessinger. That the CI configuration YAML would specify some flags explicitly, using CMAKE_CUDA_FLAGS.

Yes, it's a bit fragile, but what's going on in this PR is quite fragile as well.

[stephenswat]

Setting CMAKE_CUDA_FLAGS is high-key bad because it causes CMake to – silently, of course – ignore whatever is set in the CUDAFLAGS environment variable though... It's strictly more fragile than this approach as far as I am concerned but okay, I'll change it.

Edit: it also causes CMake to ignore it's own toolchain-specific default flags.

[stephenswat]

Updated!

[krasznaa]

Unfortunately you are correct that setting CMAKE_CUDA_FLAGS makes the configuration ignore the $CUDAFLAGS environment variable. 😦 Which can indeed cause some problems. 😦

I believe that in the single job in which we need to force -std=c++20 on CUDA, it's fine to have $CUDAFLAGS ignored. So, just since this code setup is simpler, let's go with this anyway.

P.S. We will need to test C++20 with SYCL as well later on, but it's fine not to worry about that in this PR.