[clang] Add test for CWG360

P1787: "CWG360 is resolved by applying access control to using-declarations."
[[ http://eel.is/c++draft/class.access#general-4.sentence-3 | class.access.general#4 ]]: "When a using-declarator is named, access control is applied to it, not to the declarations that replace it."

Reviewed By: #clang-language-wg, aaron.ballman

Differential Revision: https://reviews.llvm.org/D139090