[lld-macho] Stabilize symbol order from InterfaceFile
llvm/include/llvm/TextAPI/InterfaceFile.h SymbolsSet uses a
llvm::DenseMap<SymbolsMapKey, Symbol *>, which has a non-deterministic
order. For now, stabilize the order in lld since it is much simpler
than refactoring InterfaceFile.h.
