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

[Carlo Cabrera via llvm-commits]

carlocab wrote:

> Does it fire with any other build config when run against the current test set ?

It should fire with GCC, except GCC crashes before the `assert` is reached.

> BTW the suggestion about modifying `lld/test/MachO/alias-symbols.s` was to have it trigger the assert with `Clang/libc++` with the current test set.

Yes, understood, but I'm still looking into the right way to modify `alias-symbols.s`. As mentioned [above](https://github.com/llvm/llvm-project/pull/120737#issuecomment-2557400163), tips appreciated!

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