sycl::target::host_task is not implemented

[aleksmesh]

In SYCL 2020 sycl::target::host_task is introduced (4.7.6.9. Buffer accessor for commands) but not implemented.

To Reproduce

#include <sycl.hpp>

int main() {
  auto host_task = sycl::target::host_task;
}

To build:
clang++ -fsycl ./sample_host_task_target.cpp -o sample

Output:

./sample_host_task_target.cpp:4:34: error: no member named 'host_task' in 'sycl::access::target'
  auto host_task = sycl::target::host_task;
                   ~~~~~~~~~~~~~~^
1 error generated.

Environment details
Compiler version: from container d82412764b4f228f29c518e4407a87c68e8652dc

[mmoadeli]

Done in #8429