[Object] Avoid repeated map lookups (NFC) (#110201)

Author: kazutakahirata

llvm/lib/Object/ArchiveWriter.cpp

@@ -754,9 +754,8 @@ static Expected<std::vector<unsigned>> getSymbols(SymbolicFile *Obj,
       raw_string_ostream NameStream(Name);
       if (Error E = S.printName(NameStream))
         return std::move(E);
-      if (Map->find(Name) != Map->end())
+      if (!Map->try_emplace(Name, Index).second)
         continue; // ignore duplicated symbol
-      (*Map)[Name] = Index;
       if (Map == &SymMap->Map) {
         Ret.push_back(SymNames.tell());
         SymNames << Name << '\0';