[clang-tools-extra] [clangd] Add config option to allow detection of unused system headers (PR #87208)

[kadir çetinkaya via cfe-commits]

https://github.com/kadircet approved this pull request.

thanks a lot, LGTM!

it'd be great if you can also follow up with a change to https://github.com/llvm/clangd-www/blob/main/config.md

https://github.com/llvm/llvm-project/pull/87208