[clang-tools-extra] [clangd] Add tweak to override pure virtuals (PR #139348)

[kadir çetinkaya via cfe-commits]

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

thanks and sorry for not getting back to this for so long. i don't think i'll get to follow up and looks like others are also busy.

i have some usability concerns like:
- Generating method bodies in class definitions instead of out-of-line in implementation files.
- Providing implementations for all missing pure virtuals, without giving user a control might be noisy at times.
- Not providing implementations for some overriden virtuals is also something i'd miss.

But I guess most of these are improvements we can build up-on here. AFAICT, `::prepare` is both performance and resilient and we got the docs to figure out intent and fix bugs if need be. So thins LGTM as well.

https://github.com/llvm/llvm-project/pull/139348