Alternative implementation using P3074 when compiler support is available

[wusatosi]

trivial unions (was std::uninitialized<T>) introduces a standard way to provide constexpr friendly uninitialized storage.

There's wording changes related to inplace_vector included in that paper as well.

See: https://wg21.link/P3074

[wusatosi]

LLVM issue tracker: llvm/llvm-project#127601

[wusatosi]

Looks like there's issues with trivial union.

See: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3726r0.html