[basic.scope.scope] Avoid hard-to-read except...unless construction.

Factor out a name for the "unless" condition to avoid the double-negative.

Author: zygoloid

source/basic.tex

@@ -938,7 +938,11 @@
 function, or
 function template, or
 \item
-each declares a function or function template, except when
+each declares a function or function template
+and they do not declare corresponding overloads.
+\end{itemize}
+Two function or function template declarations declare
+\defn{corresponding overloads} if:
 \begin{itemize}
 \item
 both declare functions with the same non-object-parameter-type-list,
@@ -955,7 +959,6 @@
 \grammarterm{template-head}s and
 trailing \grammarterm{requires-clause}s (if any).
 \end{itemize}
-\end{itemize}
 \begin{note}
 Declarations can correspond even if neither binds a name.
 \begin{example}