[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

clang-tools-extra/clang-tidy/tool/run-clang-tidy.py

@@ -173,6 +173,9 @@ def run_tidy(args, tmpdir, build_path, queue, lock, failed_files):
     proc = subprocess.Popen(invocation, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
     output, err = proc.communicate()
     if proc.returncode != 0:
+      if proc.returncode < 0:
+        msg = "%s: terminated by signal %d\n" % (name, -proc.returncode)
+        err += msg.encode('utf-8')
       failed_files.append(name)
     with lock:
       sys.stdout.write(' '.join(invocation) + '\n' + output.decode('utf-8'))