[lld] [llvm] [lld-macho] Implement ObjC category merging (-objc_category_merging) (PR #82928)

[Vincent Lee via llvm-commits]

https://github.com/thevinster approved this pull request.

Mostly mechanical changes, otherwise seems fine to me if things are working. 

https://github.com/llvm/llvm-project/pull/82928