[clang-tools-extra] [clangd] Support outgoing calls in call hierarchy (PR #77556)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

@kadircet perhaps you might be able to pick up this review?

Or, barring 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?

https://github.com/llvm/llvm-project/pull/77556