[clang-tidy] Avoid repeated hash lookups (NFC) (#109373)

Author: kazutakahirata

clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp

@@ -116,9 +116,8 @@ void ExpandModularHeadersPPCallbacks::handleModuleFile(
   if (!MF)
     return;
   // Avoid processing a ModuleFile more than once.
-  if (VisitedModules.count(MF))
+  if (!VisitedModules.insert(MF).second)
     return;
-  VisitedModules.insert(MF);
 
   // Visit all the input files of this module and mark them to record their
   // contents later.