[expr.const] Re-apply CWG2909

Author: frederick-vs-ja

source/expressions.tex

@@ -7608,7 +7608,7 @@
 \pnum
 A constant-initializable variable is \defn{constant-initialized}
 if either it has an initializer or
-its default-initialization results in some initialization being performed.
+its type is const-default-constructible\iref{dcl.init.general}.
 \begin{example}
 \begin{codeblock}
 void f() {