I was honestly a bit surprised that we didn't do this before. This
patch is to handle "-" as the stdout so that if you pass -o - to
lld, for example, then it write an output to stdout instead of file -.
I thought that we might want to handle this at a higher level than
FileOutputBuffer, because if we land this patch, we can no longer
create a file whose name is - (there's a workaround though; you can
pass ./- instead of -). However, because raw_fd_ostream already
handle - as a special name, I think it's okay and actually more
consistent to handle - as a special name in FileOutputBuffer as well.