Preserve execution space in HostMirror if possible

[masterleinad]

Fixes #6310.

[masterleinad]

It turns out that doing

using HostMirror = View<
      typename traits::non_const_data_type, typename traits::array_layout,
      Device<std::conditional_t<
                 SpaceAccessibility<typename traits::execution_space,
                                    typename traits::host_mirror_space::
                                        memory_space>::accessible,
                 typename traits::execution_space, DefaultHostExecutionSpace>,
             typename traits::host_mirror_space::memory_space>,
      typename traits::hooks_policy>;

isn't compatible with the DualView implementation (for SharedSpace/CudaUVMSpace) that detects the View to work on based on a full match of the device derived from HostMirror. The last commit instead only keeps the execution space if it can access HostSpace.