[expr.const] Definition domain fixes for "usable in constant expressions" (#8670)

Also improves the indexing.

Author: hubert-reinterpretcast

source/expressions.tex

@@ -9190,9 +9190,10 @@
 it has reference or non-volatile const-qualified integral or enumeration type.
 
 \pnum
-A constant-initialized potentially-constant variable $V$ is
-\defn{usable in constant expressions} at a point $P$ if
-$V$'s initializing declaration $D$ is reachable from $P$ and
+A variable $V$ is
+\defnx{usable in constant expressions}{usable in constant expressions!variable} at a point $P$ if
+$V$ is constant-initialized and potentially-constant,
+$V$'s initializing declaration $D$ is reachable from $P$, and
 \begin{itemize}
 \item $V$ is constexpr,
 \item $V$ is not initialized to a TU-local value, or
@@ -9221,7 +9222,9 @@
 \end{itemize}
 
 \pnum
-An object or reference is \defn{usable in constant expressions} at point $P$
+An object or reference is
+\defnx{usable in constant expressions}{usable in constant expressions!object or reference}
+at point $P$
 if it is an object or reference
 that is potentially usable in constant expressions at $P$ and
 is constexpr-representable at $P$.