[llvm] [libc++] Ensure that `std::expected` has no tail padding (PR #69673)

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@@ -909,11 +850,19 @@ class expected {
         std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
 
+    template <class _Union>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
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philnik777 wrote:

I think we should move the `construct_at`s outside the union and instead `construct_at` the `__expected_repr`.

https://github.com/llvm/llvm-project/pull/69673