Add .gitignore file in .cache/clangd/index (#170003)
This solves a common issue where users have to manually add the
`.cache/clangd/index/` folder to their `.gitignore`. I got this idea
from [ruff](https://github.com/astral-sh/ruff), which creates
`.ruff_cache/.gitignore` and it would greatly improve the user
experience for everyone without requiring per-computer configurations
and without any significant cost.
