[clang-tools-extra] Add --project-root to clangd (PR #155905)

[via cfe-commits]

Dominicentek wrote:

1. I've attached my clangd log below.
2. Zed only lets you configure CLI options, environment variables and path to the binary. I'm using `~/.config/clangd/config.yaml` to configure the fallback flags.

[clangd.log](https://github.com/user-attachments/files/23695777/clangd.log)


https://github.com/llvm/llvm-project/pull/155905