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

[via llvm-commits]

alx32 wrote:

> the assertion I just added does _not_ fire with Apple Clang/libc++ even if I drop the `s/</>/` change from the first commit here.

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


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