Support: Functions for writing endian specific data to streams.

This adds a new header, EndianStream.h, which supplies an adaptor for
writing endian specific data to a raw_ostream.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205032 91177308-0d34-0410-b5e6-96231b3b80d8