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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

@sam-mccall review ping :)

I would particularly appreciate feedback on whether I should plan to set aside some time to implement the "deep lookup optimization" (from [this comment](https://reviews.llvm.org/D93829#4258101)), or whether the 2.5% increase in index memory usage (which I've achieved by implementing everything _except_ the deep lookup optimization from that comment) is acceptable.

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