[Serialization] Avoid repeated hash lookups (NFC) (#126429)

Author: kazutakahirata

clang/lib/Serialization/ASTReader.cpp

@@ -645,10 +645,11 @@ collectMacroDefinitions(const PreprocessorOptions &PPOpts,
 
     // For an #undef'd macro, we only care about the name.
     if (IsUndef) {
-      if (MacroNames && !Macros.count(MacroName))
+      auto [It, Inserted] = Macros.try_emplace(MacroName);
+      if (MacroNames && Inserted)
         MacroNames->push_back(MacroName);
 
-      Macros[MacroName] = std::make_pair("", true);
+      It->second = std::make_pair("", true);
       continue;
     }
 
@@ -661,9 +662,10 @@ collectMacroDefinitions(const PreprocessorOptions &PPOpts,
       MacroBody = MacroBody.substr(0, End);
     }
 
-    if (MacroNames && !Macros.count(MacroName))
+    auto [It, Inserted] = Macros.try_emplace(MacroName);
+    if (MacroNames && Inserted)
       MacroNames->push_back(MacroName);
-    Macros[MacroName] = std::make_pair(MacroBody, false);
+    It->second = std::make_pair(MacroBody, false);
   }
 }