Index: wasm/Config.h
===================================================================
--- wasm/Config.h
+++ wasm/Config.h
@@ -11,14 +11,13 @@
 #define LLD_WASM_CONFIG_H
 
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/BinaryFormat/Wasm.h"
 
 #include "Symbols.h"
 
 using llvm::wasm::WasmGlobal;
 
-#include <set>
-
 namespace lld {
 namespace wasm {
 
@@ -38,7 +37,7 @@
   llvm::StringRef OutputFile;
   llvm::StringRef Sysroot;
 
-  std::set<llvm::StringRef> AllowUndefinedSymbols;
+  llvm::StringSet<> AllowUndefinedSymbols;
   std::vector<llvm::StringRef> SearchPaths;
   std::vector<std::pair<Symbol *, WasmGlobal>> SyntheticGlobals;
 };