[clang] Mark CWG2331 as N/A

[[https://wg21.link/p1787 | P1787]]: CWG2331 is resolved by defining lookup from complete-class contexts and out-of-line member definitions.
Wording: The declaration set is the result of a single search in the scope of C for N from immediately after the class-specifier of C if P is in a complete-class context of C or from P otherwise. ([class.member.lookup]/4)

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D148260