If a clang-tidy child process exits with a signal then run-clang-tidy will exit
with an error but there is no hint why in the output, since the clang-tidy
doesn't log anything and may not even have had the opportunity to do so
depending on the signal used.
`subprocess.CompletedProcess.returncode` is the negative signal number in this
case.
I hit this in a CI system where the parallelism used exceeded the RAM assigned
to the container causing the OOM killer to SIGKILL clang-tidy processes.
Reviewed By: sylvestre.ledru
Differential Revision: https://reviews.llvm.org/D99081
f6ba03584b