[clang-tools-extra] [clangd] Update XRefs to support overriden ObjC methods (PR #127109)

[kadir çetinkaya via cfe-commits]

https://github.com/kadircet requested changes to this pull request.

can you also add some tests to `SymbolCollectorTests.cpp` to make sure we capture `overriddenby` relationships relevant to your use cases?

index tests in `XRefsTests.cpp` uses rather dynamic-index hence can have different characteristics then you'd get for the static/global index

https://github.com/llvm/llvm-project/pull/127109