Reapply "OnDiskHashTable: Use Endian.h to read little endian ostreams"

Reapply "OnDiskHashTable: Use Endian.h to read little endian ostreams"

Committed this by accident before it was done last time.

Original message:

    Rather than rolling our own functions to read little endian data
    from a buffer, we can use the support in llvm's Endian.h.

    No functional change.

llvm-svn: 205062