Adds olMemPrefetch and the OL_MEM_MIGRATION_FLAG_* enum, wired through a
new dataPrefetchImpl on the plugin interface with Level Zero and CUDA
implementations. Backends without prefetch support inherit the default
no-op.
Prefetch is a hint that migrates a USM allocation between the host and a
device so subsequent accesses on the target side are faster.
Assisted-by: Claude
Łukasz Plewa
dd824164c8