Add default constructurs to filter_iterator_impl and `filter_iterat…

…or_impl`.

Bases of `reverse_iterator` must be default-constructible. This is enforced when using `libstdc++-12` plus C++20.

Differential Revision: https://reviews.llvm.org/D141587

llvm/include/llvm/ADT/STLExtras.h

@@ -524,6 +524,8 @@ class filter_iterator_base
       BaseT::operator++();
   }
 
+  filter_iterator_base() = default;
+
   // Construct the iterator. The begin iterator needs to know where the end
   // is, so that it can properly stop when it gets there. The end iterator only
   // needs the predicate to support bidirectional iteration.
@@ -559,6 +561,8 @@ template <typename WrappedIteratorT, typename PredicateT,
 class filter_iterator_impl
     : public filter_iterator_base<WrappedIteratorT, PredicateT, IterTag> {
 public:
+  filter_iterator_impl() = default;
+
   filter_iterator_impl(WrappedIteratorT Begin, WrappedIteratorT End,
                        PredicateT Pred)
       : filter_iterator_impl::filter_iterator_base(Begin, End, Pred) {}
@@ -580,6 +584,8 @@ class filter_iterator_impl<WrappedIteratorT, PredicateT,
 public:
   using BaseT::operator--;
 
+  filter_iterator_impl() = default;
+
   filter_iterator_impl(WrappedIteratorT Begin, WrappedIteratorT End,
                        PredicateT Pred)
       : BaseT(Begin, End, Pred) {}