[lld] [lld-macho] Improve robustness of ObjC category merging (PR #112618)

[Kyungwoo Lee via llvm-commits]

https://github.com/kyulee-com approved this pull request.

LGTM. Given the possibility of invalid input to this pass, falling back to not merging seems like the right thing to do instead of asserting and crashing. By the way, does LD64 behave in the same way?

https://github.com/llvm/llvm-project/pull/112618