[clang] Mark CWG2009 as N/A

[[https://wg21.link/p1787 | P1787]]: CWG2331 is resolved by defining lookup from complete-class contexts and out-of-line member definitions. The rest of CWG2009 is resolved by making it ill-formed NDR for forward lookup outside a complete-class context to change the results (before overload resolution, to avoid differences in instantiation).
Wording: The result of the search is the declaration set of S(N, T). If it is an invalid set, the program is ill-formed. If it differs from the result of a search in T for N from immediately after the class-specifier of T, the program is ill-formed, no diagnostic required. ([class.member.lookup]/7)

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D148263