CWG2647 Fix for "needed for constant evaluation"

Also fixes NB US 17-047 (C++23 CD).

Author: burblebee

source/expressions.tex

@@ -7309,7 +7309,7 @@
 \pnum
 A variable is \defn{potentially-constant} if
 it is constexpr or
-it has reference or const-qualified integral or enumeration type.
+it has reference or non-volatile const-qualified integral or enumeration type.
 
 \pnum
 A constant-initialized potentially-constant variable $V$ is
@@ -7924,9 +7924,7 @@
 that is potentially constant evaluated, or
 
 \item
-a variable named by a potentially constant evaluated expression
-that is either a constexpr variable or
-is of non-volatile const-qualified integral type or of reference type.
+a potentially-constant variable named by a potentially constant evaluated expression.
 \end{itemize}
 
 \indextext{expression|)}