Use llvm::StringSet instead of std::set.

[ruiu]

Summary

std::set is pretty slow. We generally prefer llvm::StringSet if we don't
need an sorted set.

wasm/Config.h

 //===- Config.h -------------------------------------------------*- C++ -*-===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_WASM_CONFIG_H
 #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 {
 
 struct Configuration {
   bool AllowUndefined;
   bool Demangle;
   bool EmitRelocs;
   bool ImportMemory;
   bool Relocatable;
   bool StripAll;
   bool StripDebug;
   uint32_t GlobalBase;
   uint32_t InitialMemory;
   uint32_t MaxMemory;
   uint32_t ZStackSize;
   llvm::StringRef Entry;
   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;
 };
 
 // The only instance of Configuration struct.
 extern Configuration *Config;
 
 } // namespace wasm
 } // namespace lld
 
 #endif