allow underlying stream classes to decline buffering. After
calling SetBuffered(), re-check whether the stream is Unbuffered
in order to handle the case where the underlying stream has
declined buffering.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79362 91177308-0d34-0410-b5e6-96231b3b80d8