[clang-tools-extra] [CLANGD] [NFC] Fix proposed by sanitizer. (PR #140116)

[Nicolas van Kempen via cfe-commits]

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

LGTM!

AFAICT in this case this won't make an immediate performance difference since fields in this struct are primitives, but may in the future if the struct changes. This probably won't trigger [performance-move-const-arg](https://clang.llvm.org/extra/clang-tidy/checks/performance/move-const-arg.html) because of the `std::function` field where IIRC copying can be expensive in some cases when capturing variables.

https://github.com/llvm/llvm-project/pull/140116