[clang] Mark CWG2165 as N/A

[[ https://wg21.link/p1787 | P1787 ]]: "CWG2165 is resolved by removing the conflicting definition of it in [basic.scope]."
Wording: [basic.namespace]/p1 changed and [basic.scope.declarative] removed entirely

Reviewed By: #clang-language-wg, cor3ntin

Differential Revision: https://reviews.llvm.org/D142717