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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

I've fixed the build failure that was reported by the majority of the buildbots (which is just a new test case using a test utility function that the patch renames).

However, there is an additional build failured reported by [this buildbot](https://github.com/llvm/llvm-project/pull/77556#issuecomment-2499665944) which only applies to builds with `-DCLANGD_ENABLE_REMOTE=ON`. There is a `SymbolIndex` implementation which is only compiled in that configuration (`clang::clangd::remote::IndexClient`), which needs to be implement the new virtual method `containedRefs()` introduced by the patch.

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