[meta.trans.other] Make style of common_type and common_reference consistent

...by tagging the specification of common_reference with "Note C" and basic_common_reference "Note D".

Also remove the allowance to specialize basic_common_reference "if at least one template parameter in the specialization depends on a program-defined type"; it's redundant with "...pursuant to [namespace.std]". (This is consistent with `common_type`'s wording.)

Author: CaseyCarter

source/utilities.tex

@@ -16993,10 +16993,8 @@
  \tcode{struct}
  \hspace*{2ex}\tcode{basic_common_reference;}
  &
- A program may specialize this trait if at least one
- template parameter in the specialization depends on a program-defined
- type. If there is a member \tcode{type}, it shall be a
- \grammarterm{typedef-name}.
+ A program may specialize this trait (as specified in Note D, below). If there
+ is a member \tcode{type}, it shall be a \grammarterm{typedef-name}.
  \begin{note}
  Such specializations can be used to influence the result of
   \tcode{common_reference}.
@@ -17006,8 +17004,8 @@
 \tcode{template<class... T>} \tcode{struct common_reference;}
  &
  The member \grammarterm{typedef-name} \tcode{type} is defined or omitted
- as specified below. Each type in the parameter pack \tcode{T} shall be
- complete or (possibly \cv) \tcode{void}. \\ \rowsep
+ as specified in Note C, below. Each type in the parameter pack \tcode{T} shall
+ be complete or (possibly \cv) \tcode{void}. \\ \rowsep
 
 \indexlibrary{\idxcode{underlying_type}}%
 \tcode{template<class T>}\br
@@ -17182,9 +17180,9 @@
 No diagnostic is required for a violation of this Note's rules.
 
 \pnum
-For the \tcode{common_reference} trait applied to a parameter pack \tcode{T} of
-types, the member \tcode{type} shall be either defined or not present as
-follows:
+Note C: For the \tcode{common_reference} trait applied to a parameter pack
+\tcode{T} of types, the member \tcode{type} shall be either defined or not
+present as follows:
 \begin{itemize}
 \item If \tcode{sizeof...(T)} is zero, there shall be no member \tcode{type}.
 
@@ -17226,7 +17224,7 @@
 \end{itemize}
 
 \pnum
-Notwithstanding the provisions of \ref{meta.type.synop}, and
+Note D: Notwithstanding the provisions of \ref{meta.type.synop}, and
 pursuant to \ref{namespace.std}, a program may specialize
 \tcode{basic_common_reference<T, U, TQual, UQual>}
 for types \tcode{T} and \tcode{U} such that