raw_ostream: Lift out flush_nonempty.

 - Flush a known non-empty buffers; enforces the interface to
   flush_impl and kills off HandleFlush (which I saw no reason to be
   an inline method, Chris?).

 - Clarify invariant that flush_impl is only called with OutBufCur >
   OutBufStart.

 - This also cleary collects all places where we have to deal with the
   buffer possibly not existing.

 - A few more comments and fixing the unbuffered behavior remain in
   this commit sequence.

llvm-svn: 67057