[llvm] [dsymutil] Fix spurious warnings in MachODebugMapParser (PR #78794)

[Alpha Abdoulaye via llvm-commits]

Alpha-10000 wrote:

We want to keep the DMO as long as we're not encountering a new library.
A longer term change here would probably be to have/reuse a `Duplicates` set similarly to what's being done for OSOs ([L642](https://github.com/llvm/llvm-project/blob/main/llvm/tools/dsymutil/MachODebugMapParser.cpp#L642)), which might get rid of spurious warnings.

https://github.com/llvm/llvm-project/pull/78794