[clang-tidy] ensure run-clang-tidy reports children killed by signals

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