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

[Carlo Cabrera via llvm-commits]

carlocab wrote:

To be sure that we're not in a "works on my machine" situation, I pushed 0db5264e5918221edec3dacd2e8c0c6328769517 to ensure that the `is_partitioned` assertion triggers in CI. Sure enough, it does: https://buildkite.com/llvm-project/github-pull-requests/builds/131531

Going to revert that commit then merge.

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