This may help us reproduce a failing build when all we have is a build
log, and will become much more important in batch mode when we
/really/ need to know what ended up in what batch.
For now, this doesn't include /output/ filelists, because David's
about to mess with that code anyway to make things better around
supplementary outputs in batch mode. There is one weirdness there,
though, which is that ArgsToFrontendInputsConverter peeks at the
outputs to see whether we're doing single-threaded or multi-threaded
WMO.
This means moving the output path into SupplementaryOutputPaths, and
using the same sort of diagnostic dispatching that serialized
diagnostics use. This is part of what's needed to run the migrator
in batch mode.
