Don't allow formatted_ostream to be unbuffered, even if its underlying buffer

is.
 - The problem is that formatted_ostream forces its underlying buffer to be
   unbuffered, so if some client happens to wrap a formatted_ostream around
   something, but still use the underlying stream, then we can end up writing on
   a fully unbuffered output (which was never intended to be unbuffered).

 - This makes clang (and presumably llvm-gcc) -emit-llvm -S a mere 10x faster.

llvm-svn: 82434