[clang] Add test for CWG399

[[https://wg21.link/p1787 | P1787]]: CWG399 is resolved by explicitly appealing to the lookup for the last component of any suitable nested-name-specifier.
Wording: Otherwise, its nested-name-specifier N shall nominate a type. If N has another nested-name-specifier S, Q is looked up as if its lookup context were that nominated by S. ([basic.lookup.qual]/6.2)

CWG399 revisits a resolution to older CWG244. Our test for CWG244 covers many examples from CWG399, and it was updated in 2020 presumably aware of P1787, so I reused CWG244 test. This approach to reusing was discussed in [[https://reviews.llvm.org/D139095 | a CWG405 patch review]].

Reviewed By: #clang-language-wg, aaron.ballman, shafik

Differential Revision: https://reviews.llvm.org/D147920