[Format] Avoid repeated hash lookups (NFC) (#108794)

Author: kazutakahirata

clang/lib/Format/MacroExpander.cpp

@@ -191,9 +191,10 @@ MacroExpander::expand(FormatToken *ID,
   auto expandArgument = [&](FormatToken *Tok) -> bool {
     // If the current token references a parameter, expand the corresponding
     // argument.
-    if (Tok->isNot(tok::identifier) || ExpandedArgs.contains(Tok->TokenText))
+    if (Tok->isNot(tok::identifier))
+      return false;
+    if (!ExpandedArgs.insert(Tok->TokenText).second)
       return false;
-    ExpandedArgs.insert(Tok->TokenText);
     auto I = Def.ArgMap.find(Tok->TokenText);
     if (I == Def.ArgMap.end())
       return false;