[clang] Add test for CWG1960

P1787: "CWG1960 (currently closed as NAD) is resolved by removing the rule in question (which is widely ignored by implementations and gives subtle interactions between using-declarations)."
Wording: "In a using-declarator that does not name a constructor, every declaration named shall be accessible."

Reviewed By: #clang-language-wg, erichkeane, shafik

Differential Revision: https://reviews.llvm.org/D142381