[clang] Add test for CWG1821

[clang] Add test for CWG1821

[[https://wg21.link/p1787 | P1787]]: My clarification in [[ http://lists.isocpp.org/core/2019/06/6614.php | “Where can namespace-scope functions and variables be redeclared?” ]] is applied, resolving CWG1821.
Wording: (If the declaration is not a friend declaration:) If the id-expression in a declarator-id is a qualified-id Q, let S be its lookup context; the declaration shall inhabit a namespace scope. ([dcl.meaning]/1)

Reviewed By: shafik, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D149003