[clang-tools-extra] Add .gitignore file in .cache/clangd/index (PR #170003)

[Duncan Ogilvie via cfe-commits]

mrexodia wrote:

- Apparently there was a previous patch for this that never got merged: https://reviews.llvm.org/D130863
- Discussion thread that went nowhere: https://discourse.llvm.org/t/clangd-feature-request-automatically-generate-gitignore-in-the-cache-directory/87390
- Cargo is discussing adopting a similar approach: https://github.com/rust-lang/cargo/issues/11548#issuecomment-2735134860
- virtualenv does this https://virtualenv.pypa.io/en/latest/changelog.html#bugfixes-20-0-24
- Meson does this too
- Loads of projects add this to their .gitignore already: https://github.com/search?type=code&q=.cache%2Fclangd

https://github.com/llvm/llvm-project/pull/170003