diff --git a/llvm/include/llvm/ADT/StringMap.h b/llvm/include/llvm/ADT/StringMap.h
--- a/llvm/include/llvm/ADT/StringMap.h
+++ b/llvm/include/llvm/ADT/StringMap.h
@@ -118,36 +118,59 @@
   }
 };
 
-/// StringMapEntry - This is used to represent one value that is inserted into
-/// a StringMap.  It contains the Value itself and the key: the string length
-/// and data.
+/// StringMapEntryStorage - Holds the value in a StringMapEntry.
+///
+/// Factored out into a separate base class to make it easier to specialize.
+/// This is primarily intended to support StringSet, which doesn't need a value
+/// stored at all.
 template<typename ValueTy>
-class StringMapEntry : public StringMapEntryBase {
+class StringMapEntryStorage : public StringMapEntryBase {
 public:
   ValueTy second;
 
-  explicit StringMapEntry(size_t strLen)
+  explicit StringMapEntryStorage(size_t strLen)
     : StringMapEntryBase(strLen), second() {}
   template <typename... InitTy>
-  StringMapEntry(size_t strLen, InitTy &&... InitVals)
+  StringMapEntryStorage(size_t strLen, InitTy &&... InitVals)
       : StringMapEntryBase(strLen), second(std::forward<InitTy>(InitVals)...) {}
-  StringMapEntry(StringMapEntry &E) = delete;
-
-  StringRef getKey() const {
-    return StringRef(getKeyData(), getKeyLength());
-  }
+  StringMapEntryStorage(StringMapEntryStorage &E) = delete;
 
   const ValueTy &getValue() const { return second; }
   ValueTy &getValue() { return second; }
 
   void setValue(const ValueTy &V) { second = V; }
+};
+
+template<>
+class StringMapEntryStorage<NoneType> : public StringMapEntryBase {
+public:
+  explicit StringMapEntryStorage(size_t strLen, NoneType none = None)
+    : StringMapEntryBase(strLen) {}
+  StringMapEntryStorage(StringMapEntryStorage &E) = delete;
+
+  NoneType getValue() const { return None; }
+};
+
+/// StringMapEntry - This is used to represent one value that is inserted into
+/// a StringMap.  It contains the Value itself and the key: the string length
+/// and data.
+template<typename ValueTy>
+class StringMapEntry final : public StringMapEntryStorage<ValueTy> {
+public:
+  using StringMapEntryStorage<ValueTy>::StringMapEntryStorage;
+
+  StringRef getKey() const {
+    return StringRef(getKeyData(), this->getKeyLength());
+  }
 
   /// getKeyData - Return the start of the string data that is the key for this
   /// value.  The string data is always stored immediately after the
   /// StringMapEntry object.
   const char *getKeyData() const {return reinterpret_cast<const char*>(this+1);}
 
-  StringRef first() const { return StringRef(getKeyData(), getKeyLength()); }
+  StringRef first() const {
+    return StringRef(getKeyData(), this->getKeyLength());
+  }
 
   /// Create a StringMapEntry for the specified key construct the value using
   /// \p InitiVals.
@@ -199,7 +222,7 @@
   template<typename AllocatorTy>
   void Destroy(AllocatorTy &Allocator) {
     // Free memory referenced by the item.
-    size_t AllocSize = sizeof(StringMapEntry) + getKeyLength() + 1;
+    size_t AllocSize = sizeof(StringMapEntry) + this->getKeyLength() + 1;
     this->~StringMapEntry();
     Allocator.Deallocate(static_cast<void *>(this), AllocSize);
   }
diff --git a/llvm/include/llvm/ADT/StringSet.h b/llvm/include/llvm/ADT/StringSet.h
--- a/llvm/include/llvm/ADT/StringSet.h
+++ b/llvm/include/llvm/ADT/StringSet.h
@@ -24,8 +24,8 @@
 
   /// StringSet - A wrapper for StringMap that provides set-like functionality.
   template <class AllocatorTy = MallocAllocator>
-  class StringSet : public StringMap<char, AllocatorTy> {
-    using base = StringMap<char, AllocatorTy>;
+  class StringSet : public StringMap<NoneType, AllocatorTy> {
+    using base = StringMap<NoneType, AllocatorTy>;
 
   public:
     StringSet() = default;
@@ -37,13 +37,13 @@
 
     std::pair<typename base::iterator, bool> insert(StringRef Key) {
       assert(!Key.empty());
-      return base::insert(std::make_pair(Key, '\0'));
+      return base::insert(std::make_pair(Key, None));
     }
 
     template <typename InputIt>
     void insert(const InputIt &Begin, const InputIt &End) {
       for (auto It = Begin; It != End; ++It)
-        base::insert(std::make_pair(*It, '\0'));
+        base::insert(std::make_pair(*It, None));
     }
 
     template <typename ValueTy>
diff --git a/llvm/include/llvm/IR/Metadata.h b/llvm/include/llvm/IR/Metadata.h
--- a/llvm/include/llvm/IR/Metadata.h
+++ b/llvm/include/llvm/IR/Metadata.h
@@ -601,7 +601,7 @@
 /// These are used to efficiently contain a byte sequence for metadata.
 /// MDString is always unnamed.
 class MDString : public Metadata {
-  friend class StringMapEntry<MDString>;
+  friend class StringMapEntryStorage<MDString>;
 
   StringMapEntry<MDString> *Entry = nullptr;
 
diff --git a/llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h b/llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h
--- a/llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h
+++ b/llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h
@@ -113,7 +113,7 @@
     ShouldRestoreGlobalsLinkage = Value;
   }
 
-  void addMustPreserveSymbol(StringRef Sym) { MustPreserveSymbols[Sym] = 1; }
+  void addMustPreserveSymbol(StringRef Sym) { MustPreserveSymbols.insert(Sym); }
 
   /// Pass options to the driver and optimization passes.
   ///
diff --git a/llvm/lib/LTO/LTOCodeGenerator.cpp b/llvm/lib/LTO/LTOCodeGenerator.cpp
--- a/llvm/lib/LTO/LTOCodeGenerator.cpp
+++ b/llvm/lib/LTO/LTOCodeGenerator.cpp
@@ -151,7 +151,7 @@
 void LTOCodeGenerator::setAsmUndefinedRefs(LTOModule *Mod) {
   const std::vector<StringRef> &undefs = Mod->getAsmUndefinedRefs();
   for (int i = 0, e = undefs.size(); i != e; ++i)
-    AsmUndefinedRefs[undefs[i]] = 1;
+    AsmUndefinedRefs.insert(undefs[i]);
 }
 
 bool LTOCodeGenerator::addModule(LTOModule *Mod) {