[lld-macho] Fix assertion when two symbols at same addr have unwind info

If there are multiple symbols at the same address, our unwind info
implementation assumes that we always register unwind entries to a
single canonical symbol.

This assumption was violated by the `registerEhFrame` code.

Fixes #56570.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D130208