Resolve conflict between 2018-06 LWG Motion 19 and 2018-06 LWG Motion 21.

Motion 19 (P0769R2) adds shift_right to <algorithm> as a non-constexpr
algorithm. Motion 21 (P0879R0) asks that we mark all (non-parallel)
algorithms in <algorithm> as constexpr, but its wording doesn't list
shift_right as shift_right did not yet exist.

After consultation with LWG, resolve this state of affairs by marking
shift_right as constexpr, in line with the editing note in P0879R0.

Author: zygoloid

source/algorithms.tex

@@ -576,7 +576,7 @@
                  ForwardIterator first, ForwardIterator last,
                  typename iterator_traits<ForwardIterator>::difference_type n);
   template<class ForwardIterator>
-    ForwardIterator
+    constexpr ForwardIterator
       shift_right(ForwardIterator first, ForwardIterator last,
                   typename iterator_traits<ForwardIterator>::difference_type n);
   template<class ExecutionPolicy, class ForwardIterator>
@@ -3590,7 +3590,7 @@
 \indexlibrary{\idxcode{shift_right}}%
 \begin{itemdecl}
 template<class ForwardIterator>
-  ForwardIterator
+  constexpr ForwardIterator
     shift_right(ForwardIterator first, ForwardIterator last,
                 typename iterator_traits<ForwardIterator>::difference_type n);
 template<class ExecutionPolicy, class ForwardIterator>