rustc: make stack traces print for .span_bug/.bug.

Previously a call to either of those to diagnostic printers would defer
to the `fatal` equivalents, which explicitly silence the stderr
printing, including a stack trace from `RUST_LOG=std::rt::backtrace`.

This splits the bug printers out to their own diagnostic type so that
things work properly.

Also, this removes the `Ok(...)` that was being printed around the
subtask's stderr output.