As pointed out on a recent JIRA, crash traces don't mention what
version of Swift you were running. Usually that can be gleaned from
the path, but not always.
To display stack traces from child processes, the driver combines stdout
and stderror together on Unix. This makes the Basic implementation also
do that.
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.
