MTLHeaps on Apple Device Simulator must use private storage

[SRSaunders]

Fixes #2472

[cdavis5e]

Yet another instance where the Simulator disables Apple Silicon features on Apple Silicon GPUs for no real reason.

[SRSaunders]

Yet another instance where the Simulator disables Apple Silicon features on Apple Silicon GPUs for no real reason.

Perhaps it's because the Simulator runs on both x86_64 and arm64 hosts, and Apple wants the same behaviour on both? Or perhaps they only want to maintain one code path? Note I actually tested this PR on both host arches to make sure it was truly necessary on arm64 hosts - and it is.

[billhollings]

Thanks for catching, and thanks for the fix!