Move endian constant from Host.h to SwapByteOrder.h, prune include
Works on this dependency chain: ArrayRef.h -> Hashing.h -> --CUT-- Host.h -> StringMap.h / StringRef.h
ArrayRef is very popular, but Host.h is rarely needed. Move the IsBigEndianHost constant to SwapByteOrder.h. Clients of that header are more likely to need it.