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

[Carlo Cabrera via llvm-commits]

carlocab wrote:

> Guess the problem is to understand which comparison function to provide given that the fault is in the comparison function :-)

This comment notwithstanding, I added an assertion anyway since it seems like an improvement. Interestingly, the assertion I just added does _not_ fire with Apple Clang/libc++ even if I drop the `s/</>/` change from the first commit here.

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