master-next: Change TransformIterator to work with bidirectional iterators.

The custom DFS implementation for dominators in LLVM r307727 uses a reverse
iterator and so fails to build if SILBasicBlocks do not support that.

include/swift/Basic/STLExtras.h

@@ -392,7 +392,7 @@ makeFilterRange(Range range, Predicate pred) {
   return FilterRange<Range, Predicate>(range, pred);
 }
 
-/// An iterator that transforms the result of an underlying forward
+/// An iterator that transforms the result of an underlying bidirectional
 /// iterator with a given operation.
 ///
 /// \tparam Iterator the underlying iterator.
@@ -411,7 +411,7 @@ class TransformIterator {
   using OpTraits = function_traits<Operation>;
 
 public:
-  using iterator_category = std::forward_iterator_tag;
+  using iterator_category = std::bidirectional_iterator_tag;
   using value_type = typename OpTraits::result_type;
   using reference = value_type;
   using pointer = void; // FIXME: Should provide a pointer proxy.
@@ -438,6 +438,17 @@ public:
     return old;
   }
 
+  TransformIterator &operator--() {
+    --Current;
+    return *this;
+  }
+
+  TransformIterator operator--(int) {
+    TransformIterator old = *this;
+    --*this;
+    return old;
+  }
+
   friend bool operator==(TransformIterator lhs, TransformIterator rhs) {
     return lhs.Current == rhs.Current;
   }