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

[David Goldman via cfe-commits]

DavidGoldman wrote:

> 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

Done, added a simple inheritance test.

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