[clang] [clang] add test verifying DR242 behavior (PR #201441)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> This is blocked because [CWG507 was recently changed to open](https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html), which diverges from the clang status page ([507 was previously closed as duplicate of 260](https://clang.llvm.org/cxx_dr_status.html)).

This will be resolved shortly by #205342.

https://github.com/llvm/llvm-project/pull/201441