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.
The calls to std::construct_at might overwrite the previously set
__has_value_ flag in the case where the flag is overlapping with
the actual value or error being stored (since we use [[no_unique_address]]).
To fix this issue, this patch ensures that we initialize the
__has_value_ flag after we call std::construct_at.
Fixes#68552
