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

[via cfe-commits]

github-actions[bot] wrote:

⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
  Please turn off [Keep my email addresses private](https://github.com/settings/emails) setting in your account.
  See [LLVM Discourse](https://discourse.llvm.org/t/hidden-emails-on-github-should-we-do-something-about-it) for more information.


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