FileOutputBuffer: Handle "-" as stdout.

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, it writes 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
handles `-` as a special file name, I think it's okay and actually
consistent to handle `-` as a special name in FileOutputBuffer.

Differential Revision: https://reviews.llvm.org/D56940

llvm-svn: 351852