[meta.trans.other] Remove basic_common_reference requirement with no normative effect

...except to make some programs ill-formed NDR. Also clarify that users may *partially* specialize basic_common_reference.

Author: CaseyCarter

source/utilities.tex

@@ -17190,7 +17190,7 @@
 
 \pnum
 Note D: Notwithstanding the provisions of \ref{meta.type.synop}, and
-pursuant to \ref{namespace.std}, a program may specialize
+pursuant to \ref{namespace.std}, a program may partially specialize
 \tcode{basic_common_reference<T, U, TQual, UQual>}
 for types \tcode{T} and \tcode{U} such that
 \tcode{is_same_v<T, decay_t<T>{>}} and
@@ -17206,9 +17206,7 @@
 Moreover, \tcode{basic_common_reference<T, U, TQual, UQual>::type} shall denote
 the same type, if any, as does
 \tcode{basic_common_reference<U, T, UQual, TQual>::type}.
-A program shall not specialize \tcode{basic_common_reference} on the third or
-fourth parameters, \tcode{TQual} or \tcode{UQual}. No diagnostic is required for
-a violation of these rules.
+No diagnostic is required for a violation of these rules.
 
 \pnum
 \begin{example}