[clang-tools-extra] [clangd] Re-land "support outgoing calls in call hierarchy" (PR #117673)

[via cfe-commits]

pidgeon777 wrote:

> > I would appreciate confirmation that this will indeed be possible with the upcoming **clangd** changes.
> 
> Yes, the returned data is sufficient for clients to implement either of the above presentations.

This is great news, thank you!

https://github.com/llvm/llvm-project/pull/117673