[SYCLNATIVECPU] Re-enable host device

[PietroGhg]

This PR re-enables the host device, making it again available in user code. This is needed by the SYCL Native CPU compiler flow to accelerate SYCL on CPUs. This PR only contains changes needed to re-enable the host device and no changes related to SYCL Native CPU. The changes relative to SYCL Native CPU are contained in a separate PR, which, for ease of review, currently lives in a separate fork, see this. Once this re-enablement PR is merged in, we will re-target the SYCL Native CPU PR to this repo.

This PR in particular:

• Reverts [SYCL] Make host device inaccessible through SYCL API #6685
• Re-enables the host device in the sycl-e2e tests
• Fixes some issues encountered with sycl::stream::flush (f9f3008) and device filtering (28a6353).

[dm-vodopyanov]

@gmlueck @jbrodman, this is a PR for re-enabling host device. You may have some comments here as this violates SYCL 2020 spec. In common, do we have some plans for re-enabling host device?

[PietroGhg]

@gmlueck @jbrodman, this is a PR for re-enabling host device. You may have some comments here as this violates SYCL 2020 spec. In common, do we have some plans for re-enabling host device?

For us this was the quickest way to get kernels to run through the SYCL native CPU path, but I understand the concerns about the 2020 spec and in general maintainability of the host device, so I'm closing this PR while we work on a better solution, thank you

[uwedolinsky]

The plan was to use the host device for parts and no longer expose it to the user - so for the current work we've just re-enabled it, and in that PR we then wanted to gradually remove the bits we no longer need.