rustc hangs if dsymutil writes to stdout

As discovered in #15978, rustc hangs if dsymutil writes to stdout, because it uses `.status()` rather than `.output()` to determine whether the command completes. I don't know if this points to a general issue with `.status()` or not. Since dsymutil actually writes its errors to stdout, it might be good to also proxy the output so that the user can see that e.g. rustc generated invalid DWARF.
