OnDiskHashTable: Use EndianStream.h to write little endian ostreams

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

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.