GPU: reach global memory through the generic address space on Metal (#15936)

GPUglobalref() expanded to device, which is what the OpenCL 1 port needed. It no
longer matches the code: since the constant address space was dropped, the
objects the kernels work on are reached through a generic `this`, so a member
pointer or the address of a member is a generic pointer, and MSL will not pass
one where a device pointer is wanted.

MSL 4.1's generic address space spans device, threadgroup and thread, so leaving
the annotation off is correct for every one of these. This mirrors the OpenCL C++
branch, where GPUglobalref() is likewise empty and GPUgeneric() carries the
annotation. GPUsharedref() and GPUconstantref() stay explicit: threadgroup is
still worth pinning where it is known, and constant is not reachable through a
generic pointer at all.

Author: ktf

GPU/Common/GPUCommonDefAPI.h

@@ -166,7 +166,7 @@
   #define GPUnoexcept()
   #define GPUprivate() thread
   #define GPUgeneric()
-  #define GPUglobalref() device
+  #define GPUglobalref()
   #define GPUsharedref() threadgroup
   #define GPUprivateref() thread
   #define GPUconstexprref() GPUconstexpr()