[lldb] Avoid repeated map lookups (NFC) (#123892)

Author: kazutakahirata

lldb/source/Target/Process.cpp

@@ -1294,17 +1294,12 @@ bool Process::HasAssignedIndexIDToThread(uint64_t thread_id) {
 }
 
 uint32_t Process::AssignIndexIDToThread(uint64_t thread_id) {
-  uint32_t result = 0;
-  std::map<uint64_t, uint32_t>::iterator iterator =
-      m_thread_id_to_index_id_map.find(thread_id);
-  if (iterator == m_thread_id_to_index_id_map.end()) {
-    result = ++m_thread_index_id;
-    m_thread_id_to_index_id_map[thread_id] = result;
-  } else {
-    result = iterator->second;
-  }
+  auto [iterator, inserted] =
+      m_thread_id_to_index_id_map.try_emplace(thread_id, m_thread_index_id + 1);
+  if (inserted)
+    ++m_thread_index_id;
 
-  return result;
+  return iterator->second;
 }
 
 StateType Process::GetState() {