Before the endianness was specified on each call to read
or write of the StreamReader / StreamWriter, but in practice
it's extremely rare for streams to have data encoded in
multiple different endiannesses, so we should optimize for the
99% use case.
This makes the code cleaner and more general, but otherwise
has NFC.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296415 91177308-0d34-0410-b5e6-96231b3b80d8