Parallelize clang-tidy hook

[hx507]

Currently clang-tidy hook is running in serial, which can be slow if multiple large files are modified. Clang-tidy is trivially parallelize-able per file, so why don't we take advantage of that? Locally it speeds up a ~8 sec clang-tidy run to ~3 sec, which is bottle-necked on largest file changed -- still much faster.