Merge 2018-11 LWG Motion 10

P1006R1 Constexpr in std::pointer_traits

Fixes #2416

Author: zygoloid

source/utilities.tex

@@ -6863,7 +6863,7 @@
 
     template<class U> using rebind = U*;
 
-    static pointer pointer_to(@\seebelow@ r) noexcept;
+    static constexpr pointer pointer_to(@\seebelow@ r) noexcept;
   };
 }
 \end{codeblock}
@@ -6919,7 +6919,7 @@
 \indexlibrarymember{pointer_to}{pointer_traits}%
 \begin{itemdecl}
 static pointer pointer_traits::pointer_to(@\seebelow@ r);
-static pointer pointer_traits<T*>::pointer_to(@\seebelow@ r) noexcept;
+static constexpr pointer pointer_traits<T*>::pointer_to(@\seebelow@ r) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}