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

[Vadim D. via cfe-commits]

vvd170501 wrote:

@HighCommander4, overall it looks good, but I'd replace "enables include-cleaner checks" with "enables unused include check", because the option doesn't affect missing include check.

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