[clang-query] Parallel runner for clang-query

Similarly to [run-clang-tidy.py](https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/clang-tidy/tool/run-clang-tidy.py), should we have a parallel runner for `clang-query` as well? For big searches in big codebases.

Start by reading clang-tools-extra/clang-tidy/tool/run-clang-tidy.py, which the issue identifies as the analogous runner. Determine the desired parallel-runner scope for clang-query and how it should support searches across large codebases; done means an agreed implementation that runs clang-query searches in parallel.

Written by the indexing model from the issue text.