CommonDataFormat: place shared constants in the constant address space

MSL requires every variable at program scope, class-scope statics included, to
name an address space. GPUglobalconstexpr() expands to constexpr everywhere
except Metal, so this is a no-op for host, CUDA, HIP, OpenCL and cling.

Author: ktf

DataFormats/common/include/CommonDataFormat/AbstractRef.h

@@ -22,7 +22,6 @@
 #include <type_traits>
 #endif
 
-
 namespace o2::dataformats
 {
 
@@ -54,10 +53,10 @@ class AbstractRef
   using Src_t = decltype(AbstractRef::MVAR<NBSrc>());
   using Flg_t = decltype(AbstractRef::MVAR<NBFlg>());
 
-  static constexpr Base_t BaseMask = Base_t((((0x1U << (NBIdx + NBSrc + NBFlg - 1)) - 1) << 1) + 1);
-  static constexpr Idx_t IdxMask = Idx_t((((0x1U << (NBIdx - 1)) - 1) << 1) + 1);
-  static constexpr Src_t SrcMask = Src_t((((0x1U << (NBSrc - 1)) - 1) << 1) + 1);
-  static constexpr Flg_t FlgMask = Flg_t((((0x1U << (NBFlg - 1)) - 1) << 1) + 1);
+  static GPUglobalconstexpr() Base_t BaseMask = Base_t((((0x1U << (NBIdx + NBSrc + NBFlg - 1)) - 1) << 1) + 1);
+  static GPUglobalconstexpr() Idx_t IdxMask = Idx_t((((0x1U << (NBIdx - 1)) - 1) << 1) + 1);
+  static GPUglobalconstexpr() Src_t SrcMask = Src_t((((0x1U << (NBSrc - 1)) - 1) << 1) + 1);
+  static GPUglobalconstexpr() Flg_t FlgMask = Flg_t((((0x1U << (NBFlg - 1)) - 1) << 1) + 1);
   static constexpr int NBitsIndex() { return NBIdx; }
   static constexpr int NBitsSource() { return NBSrc; }
   static constexpr int NBitsFlags() { return NBFlg; }

DataFormats/common/include/CommonDataFormat/RangeReference.h

@@ -64,9 +64,9 @@ class RangeRefComp
   using Base = unsigned int;
 
  private:
-  static constexpr int NBitsTotal = sizeof(Base) * 8;
-  static constexpr Base MaskN = ((0x1 << NBitsN) - 1);
-  static constexpr Base MaskR = (~Base(0)) & (~MaskN);
+  static GPUglobalconstexpr() int NBitsTotal = sizeof(Base) * 8;
+  static GPUglobalconstexpr() Base MaskN = ((0x1 << NBitsN) - 1);
+  static GPUglobalconstexpr() Base MaskR = (~Base(0)) & (~MaskN);
   Base mData = 0; ///< packed 1st entry reference + N entries
   GPUhd() void sanityCheck()
   {