Reapply "OnDiskHashTable: Use EndianStream.h to write little endian ostreams"

Committed this by accident before it was done last time.

Original message:

    Rather than rolling our own functions to write little endian data
    to an ostream, we can use the support in llvm's EndianStream.h.

    No functional change.
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.