Add iOS/tvOS destinations

[JoeMatt]

Contingent on https://github.com/stuartcarnie/SwiftSPIRV-Cross/pull/2

In the meantime, I'm pointing to my "fork" here.

https://github.com/Provenance-Emu/SwiftSPIRV-Cross/tree/provenance

Provenance is looking into leveraging this framework for additional shader support in Metal.

Changes

1. Forked SwiftSPIRV to add ios/ipados/tvos destinations
2. Added ios/ipados/tvos destinations to this workspace.
3. Fixed (i think) code for NSRect and managedMemory on iOS that is unavailable.

3 was quick and dirty to get it to build. Not sure what the side effects may be, if any.

Notes

I clamped the iOS/tvOS versions to 14. It would probably work on 13 but I don't need it that far back.