[clang] Add test for CWG418

P1787: //[[ https://cplusplus.github.io/CWG/issues/418.html | CWG418 ]] is resolved by trivial rephrasing along with that necessary for the new using-declaration interpretation.//
Wording: see changes to [dcl.fct.default]/9 and [over.match.best]/4.

[over.match.best]/4 includes [[ https://eel.is/c++draft/over.match.best#general-example-8 | an example ]] that is not properly diagnosed by Clang.

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D139429