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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205062 91177308-0d34-0410-b5e6-96231b3b80d8