Make AsmWriter more careful with formatted_raw_ostream so that

Make AsmWriter more careful with formatted_raw_ostream so that
it doesn't leave the underlying stream in unbuffered mode when
the stream was originally buffered.

Also, change WriteAsOperand back to plain raw_ostream. This
lets it work for either formatted_raw_ostream or plain
raw_ostream, so that it doesn't have to force a buffer flush
on a plain raw_ostream.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78837 91177308-0d34-0410-b5e6-96231b3b80d8