[utility.as.const] Revert changes of as_const

Author: frederick-vs-ja

source/utilities.tex

@@ -63,7 +63,7 @@
 
   // \ref{utility.as.const}, \tcode{as_const}
   template<class T>
-    constexpr const T& as_const(T& t) noexcept;
+    constexpr add_const_t<T>& as_const(T& t) noexcept;
   template<class T>
     void as_const(const T&&) = delete;
 
@@ -495,7 +495,7 @@
 
 \indexlibraryglobal{as_const}%
 \begin{itemdecl}
-template<class T> constexpr const T& as_const(T& t) noexcept;
+template<class T> constexpr add_const_t<T>& as_const(T& t) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}