[clang] Add test for CWG2007

[[https://wg21.link/p1787 | P1787]]: CWG2007 is resolved by skipping unqualified lookup for operators that must be member functions.
Wording: For the operators =, [], or ->, the set of non-member candidates is empty; otherwise, it includes the result of the unqualified lookup for operator@... ([over.match.oper]/3)

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D147839