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.

llvm-svn: 375316