[depr.static.constexpr] Cross-reference core clauses for deprecated feature (#6448)

Author: AlisdairM

source/basic.tex

@@ -182,7 +182,7 @@
 \item
 it declares a static data member outside a class definition
 and the variable was defined within the class with the \keyword{constexpr}
-specifier (this usage is deprecated; see \ref{depr.static.constexpr}),
+specifier\iref{class.static.data} (this usage is deprecated; see \ref{depr.static.constexpr}),
 \item
 \indextext{declaration!class name}%
 it is an \grammarterm{elaborated-type-specifier}\iref{class.name},

source/future.tex

@@ -140,7 +140,8 @@
 
 \pnum
 For compatibility with prior revisions of \Cpp{}, a \keyword{constexpr}
-static data member may be redundantly redeclared outside the class with no initializer.
+static data member may be redundantly redeclared outside the class with no
+initializer\iref{basic.def,class.static.data}.
 This usage is deprecated.
 \begin{example}
 \begin{codeblock}