[lldb] Avoid repeated map lookups (NFC) (#112823)

Author: kazutakahirata

lldb/source/Commands/CommandObjectMultiword.cpp

@@ -102,11 +102,9 @@ llvm::Error CommandObjectMultiword::LoadUserSubcommand(
 
   std::string str_name(name);
 
-  auto pos = m_subcommand_dict.find(str_name);
-  if (pos == m_subcommand_dict.end()) {
-    m_subcommand_dict[str_name] = cmd_obj_sp;
+  auto [pos, inserted] = m_subcommand_dict.try_emplace(str_name, cmd_obj_sp);
+  if (inserted)
     return llvm::Error::success();
-  }
 
   const char *error_str = nullptr;
   if (!can_replace)
@@ -117,7 +115,7 @@ llvm::Error CommandObjectMultiword::LoadUserSubcommand(
   if (error_str) {
     return llvm::createStringError(llvm::inconvertibleErrorCode(), error_str);
   }
-  m_subcommand_dict[str_name] = cmd_obj_sp;
+  pos->second = cmd_obj_sp;
   return llvm::Error::success();
 }