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

[via llvm-commits]

yulingtianxia wrote:

[L308-L309](https://github.com/llvm/llvm-project/blob/main/llvm/tools/dsymutil/MachODebugMapParser.cpp#L308-L309) of `MachODebugMapParser::switchToNewLibDebugMapObject` may have the same issue @JDevlieghere 


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