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

[via llvm-commits]

alx32 wrote:

> 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?

Not sure about LD64 behavior. The issue here is very specific to the internals of LLD pointing to a StringPiece rather than a Symbol. LD64's implementation revolves around Atom's so the issue does not translate over. 

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