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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> @HighCommander4 Ping. Could you review this PR or suggest other reviewers, please?

I'm not very familiar with include-cleaner, but I added some reviewers who are.

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