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

include/llvm/Support/FormattedStream.h

@@ -105,10 +105,15 @@ namespace llvm
       // own buffering, and it doesn't need or want TheStream to do another
       // layer of buffering underneath. Resize the buffer to what TheStream
       // had been using, and tell TheStream not to do its own buffering.
+      //
+      // If the underlying stream is unbuffered, just use its preferred buffer
+      // size. We can't treat this as an honest wish for unbuffered output,
+      // because it could very well be a stream we previously forced to be
+      // unbuffered.
       if (size_t BufferSize = TheStream->GetBufferSize())
         SetBufferSize(BufferSize);
       else
-        SetUnbuffered();
+        SetBuffered();
       TheStream->SetUnbuffered();
 
       Scanned = 0;