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

[David Goldman via cfe-commits]

DavidGoldman wrote:

> i am not sure if it does, e.g. https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/DeclObjC.cpp#L1268-L1273 stops at the first overridden method.
> 
> maybe you can add some unittests for this as well ?

Done, you're right, so fixed that as well.

> i think using go-to-definition for such interactions is a more common workflow (also go-to-impl is not necessarily implemented by all editors). so I think there's merit in making sure that also works (though I won't block this patch on it, as it's not a regression. but I think it warrants at least a FIXME)

Added a FIXME

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