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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205061 91177308-0d34-0410-b5e6-96231b3b80d8