[clang] Add test for CWG607

[[https://wg21.link/p1787 | P1787]]: CWG607 is resolved by looking up unqualified names in a mem-initializer-id from outside the parameter scope.
Wording: Lookup for an unqualified name in a mem-initializer-id ignores the constructor’s function parameter scope. ([class.base.init]/2)

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D147590