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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> > @kadircet perhaps you might be able to pick up this review?
> > Or, in the absence of a full review, your opinion on the directional question in [this comment](https://reviews.llvm.org/D93829#4654786) would be appreciated as well:
> > > how would you feel about proceeding with the patch in its current state, with the memory usage increase brought down from 8.2% to 2.5% thanks to the combination of the simple lookup optimization + RefKind filtering, and leaving the "deep lookup optimization" to be explored in a future change?
> 
> I'd definitely prefer the one we discussed in the original review, but I don't think perfect needs to be enemy of the good, we can surely optimize data structures here going forward if needed.

I filed https://github.com/clangd/clangd/issues/2264 as a follow-up to track implementation of the "deep lookup optimization".

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