Make some methods const.  The only interesting change here is that
it changes raw_fd_ostream::preferred_buffer_size to return zero on
a scary stat failure instead of setting the stream to an error state.
This method really should not mutate the stream.

llvm-svn: 91740

means that raw_ostream no longer has to #include <iosfwd>.  Nothing in llvm
should use raw_os_ostream.h, but llvm-gcc and some unit tests do.

llvm-svn: 79886

llvm-svn: 45418

llvm-svn: 44597

llvm-svn: 44582