[bit.cast] Update cross-reference into [expr.const] following 98a668e.

The editorial change was made after the wording of CWG 2765 was written.

Addresses editorial review committee feedback.

Author: tkoeppe

source/utilities.tex

@@ -16369,7 +16369,7 @@
 \pnum
 \constantwhen
 Neither \tcode{To} nor \tcode{From}
-has constexpr-unknown representation\iref{expr.const}.
+has constexpr-unknown representation\iref{expr.const.core}.
 
 \pnum
 \returns