[clang] Add test for CWG2396

Also mark [[ https://wg21.link/cwg1291 | CWG1291 ]] as "na".

[[ https://wg21.link/p1787 | P1787 ]]: CWG1291 and CWG2396 are resolved by explicitly specifying how to look up names in a conversion-type-id.
Wording: see changes to [basic.lookup.unqual]/5 and [basic.lookup.qual]/2.

"Calling a conversion function" example in [[ https://wg21.link/p1787 | P1787 ]] is also relevant.

Differential Revision: https://reviews.llvm.org/D142316