Implement UniqueToken for SYCL

[masterleinad]

This also allows running all container tests.

AFAICT, the concurrency calculation here is an upper limit for Intel GPUs and should match the value we get for Nvidia GPUs. As usual, I checked that this pull request also passes on Intel GPUs.
We still don't have a good implementation for clock_tick so the UniqueToken might be pretty slow.

[masterleinad]

Hmm.. This passes with an older SYCL-CUDA release but not with the one we are now using for testing. I'll try to check which one broke or at least if I can find one that we can compile and that works also for this pull request.

[masterleinad]

This should work now. I'm still not quite sure about the internal compiler errors but they are related to files that shouldn't matter too much for the SYCL CI (except maybe for the header self-containment tests). I will, of course, try to dig a little more but I think the status here is acceptable for the release. As mentioned before, the tests are all passing without the workarounds for Host and Intel GPU; only the SYCL CUDA backend causes all the problems here.

One of the problematic files was TestHostBarrier.cpp which turned out to be empty. I'm happy to create another pull request for that.

Other than that, I needed to rebase on top of #3815 to make all tests pass.

[masterleinad]

Retest this please.

[Rombur]

This looks fine with the caveat that I don't know SYCL...

[masterleinad]

Rebased. Still depends on #3815.

[dalg24]

Please rebase