[clang] Add test for CWG191

Also mark CWG1200 as "na."

[[https://wg21.link/p1787 | P1787]]: CWG191 and CWG1200 are resolved by defining unqualified lookup in terms of every enclosing scope.
Wording: If no declarations are found, the results of the unqualified search are the results of an unqualified search in the parent scope of S, if any, from P. ([basic.lookup.unqual]/2)

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D147530