[NFC] use StringSet::insert(iter, iter) instead for loop to insert (#69851)

Author: HerrCai0907

clang/lib/ARCMigrate/ObjCMT.cpp

@@ -123,9 +123,7 @@ class ObjCMigrateASTConsumer : public ASTConsumer {
         NSIntegerTypedefed(nullptr), NSUIntegerTypedefed(nullptr),
         Remapper(remapper), FileMgr(fileMgr), PPRec(PPRec), PP(PP),
         IsOutputFile(isOutputFile), FoundationIncluded(false) {
-    // FIXME: StringSet should have insert(iter, iter) to use here.
-    for (const std::string &Val : AllowList)
-      AllowListFilenames.insert(Val);
+    AllowListFilenames.insert(AllowList.begin(), AllowList.end());
   }
 
 protected: