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.

llvm-svn: 78837