[defns.undefined] Move cross-reference to [expr.const]

Fixes NB JP 003 (C++20 DIS)

Author: jensmaurer

source/intro.tex

@@ -366,8 +366,8 @@
 diagnostic message), to terminating a translation or execution (with the
 issuance of a diagnostic message). Many erroneous program constructs do
 not engender undefined behavior; they are required to be diagnosed.
-Evaluation of a constant expression never exhibits behavior explicitly
-specified as undefined in \ref{intro} through \ref{cpp}\iref{expr.const}.
+Evaluation of a constant expression\iref{expr.const} never exhibits behavior explicitly
+specified as undefined in \ref{intro} through \ref{cpp}.
 \end{defnote}
 
 \indexdefn{behavior!unspecified}%