TPC: place shared constants in the Metal constant address space

[ktf]

MSL requires namespace-scope constexpr to live in the constant address space.
Generic pointers do not help here: constant is explicitly not among the
address spaces MSL 4.1 makes generic, so the annotation is required whatever
the language version. Guarded on METAL, so no other build changes.

[ktf]

@davidrohr an alternative would be to have a GPUconstexprconstant() macro which expands to constant constexpr on mac and just constexpr (and not constexpr __constant__) everywhere else.

[davidrohr]

So MacOS does not support constexpr variables, when they are not constant?

[ktf]

As far as I understand it, only if they are in the (C++) global scope. They work fine if they are inside a function.

[ktf]

If you prefer we can have a GPUglobalconstant().

[davidrohr]

I would add a GPUglobalconstexpr() macro, we might need the same in other places.

[ktf]

@davidrohr done