[variant.helper] Use 'struct' for variant_size and variant_alternative

Fixes NB JP 005 and JP 006 (C++20 DIS)

Author: jensmaurer

source/utilities.tex

@@ -4811,7 +4811,7 @@
 
 \indexlibraryglobal{variant_size}%
 \begin{itemdecl}
-template<class T> class variant_size<const T>;
+template<class T> struct variant_size<const T>;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4831,7 +4831,7 @@
 
 \indexlibraryglobal{variant_alternative}%
 \begin{itemdecl}
-template<size_t I, class T> class variant_alternative<I, const T>;
+template<size_t I, class T> struct variant_alternative<I, const T>;
 \end{itemdecl}
 
 \begin{itemdescr}