[libc++][test] Avoid C++23 Core features that MSVC lacks (#73438)

Found while running libc++'s test suite with MSVC's STL, where we use
both Clang and MSVC's compiler.

libc++'s test suite has started using some C++23 Core Language features
that MSVC hasn't implemented yet. When avoiding these features costs
just a tiny bit of syntax, I would like to ask libc++ to consider making
that change in the interest of practical portability, at least until
MSVC catches up. If there's no desire to do so, then I could skip the
affected tests, but that would make me a slightly sad kitty.