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

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

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D56854

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351566 91177308-0d34-0410-b5e6-96231b3b80d8

Author: ilya-biryukov

include/llvm/Support/Registry.h

@@ -81,17 +81,17 @@ namespace llvm {
 
     /// 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