Cancelling the macOS process-ownership helper build left swiftc object files
in TMPDIR. Node's execFile sends SIGTERM on abort and rejects before the
child exits, and swift-driver runs each frontend job in its own process
group: on SIGTERM the driver exits while a job keeps writing, so REA removed
its root while objects were still being written outside and inside it.
Add an opt-in `stopSignal` to execFileOutput that stops the process with the
caller's signal on abort or timeout and settles only after it exits. Swift
helper builds stop swiftc with SIGINT, on which swift-driver stops its jobs
and deletes their objects, and point TMPDIR inside the root cleanup removes.
Fixes#1173
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
morluto