[class] Add index cross-references to subentries under "class"

Add index cross-references so "trival class" and "trivially copyable
class" refer to "class, trivial" and "class, trivially copyable"
respectively.

Also add a missing \pnum before the paragraph defining trivial classes.

source/classes.tex

@@ -124,8 +124,8 @@
 Aggregates of class type are described in~\ref{dcl.init.aggr}.
 \end{note}
 
-\indextext{trivial class|see{class, trivial}}%
 \pnum
+\indextext{trivially copyable class|see{class, trivially copyable}}%
 A \defnx{trivially copyable class}{class!trivially copyable} is a class:
 
 \begin{itemize}
@@ -137,15 +137,17 @@
 \item that has a trivial, non-deleted destructor\iref{class.dtor}.
 \end{itemize}
 
+\pnum
+\indextext{trivial class|see{class, trivial}}%
 A \defnx{trivial class}{class!trivial} is a class that is trivially copyable and
 has one or more default constructors\iref{class.ctor},
 all of which are either trivial or deleted and
 at least one of which is not deleted.
 \begin{note} In particular, a trivially copyable or trivial class does not have
 virtual functions or virtual base classes.\end{note}
 
-\indextext{standard-layout|see{class, standard-layout}}%
 \pnum
+\indextext{standard-layout|see{class, standard-layout}}%
 A class \tcode{S} is a \defnx{standard-layout class}{class!standard-layout} if it:
 \begin{itemize}
 \item has no non-static data members of type non-standard-layout class