[lld] [lld][MachO] Fix symbol insertion in `transplantSymbolsAtOffset` (PR #120737)

[Carlo Cabrera via llvm-commits]

carlocab wrote:

I hadn't looked at `alias-symbols.s` since that wasn't one of the failing tests flagged at #120559, but the approach you suggest sounds promising.

I'll give it a try, but would appreciate as many hints as possible. Not really familiar with how symbols are laid out in x86 assembly.

https://github.com/llvm/llvm-project/pull/120737