[lld] [lld-macho] Fix category merging category map non-determinism (PR #91159)

[Kyungwoo Lee via llvm-commits]

https://github.com/kyulee-com commented:

Have you thought about using `llvm::MapVector` (instead of `llvm::DenseMap`) which should preserve the input/insertion order to the map, which you seem want here?

https://github.com/llvm/llvm-project/pull/91159