[Support] Implement llvm::Registry::iterator via llvm_iterator_facade

[ilya-biryukov]

Summary

Among other things, this allows using STL algorithms like 'find_if' over
llvm::Registry.

[ilya-biryukov]

• Move to the monorepo

llvm/trunk/include/llvm/Support/Registry.h

public:
     /// This function is exported by the executable and called by the plugin to
     /// add a node to the executable's registry. Therefore it's not defined here
     /// to avoid it being instantiated in the plugin and is instead defined in
     /// the executable (see LLVM_INSTANTIATE_REGISTRY below).
     static void add_node(node *N);
 
     /// Iterators for registry entries.
     ///
-    class iterator {
+    class iterator
+        : public llvm::iterator_facade_base<iterator, std::forward_iterator_tag,
+                                            const entry> {
       const node *Cur;
 
     public:
       explicit iterator(const node *N) : Cur(N) {}
 
       bool operator==(const iterator &That) const { return Cur == That.Cur; }
-      bool operator!=(const iterator &That) const { return Cur != That.Cur; }
       iterator &operator++() { Cur = Cur->Next; return *this; }
       const entry &operator*() const { return Cur->Val; }
-      const entry *operator->() const { return &Cur->Val; }
     };
 
     // begin is not defined here in order to avoid usage of an undefined static
     // data member, instead it's instantiated by LLVM_INSTANTIATE_REGISTRY.
     static iterator begin();
     static iterator end()   { return iterator(nullptr); }
 
     static iterator_range<iterator> entries() {