[libcxx] [libcxx] makes `expected` trivially assignable when both members are … (PR #74768)

[Jan Kokemüller via libcxx-commits]

jiixyj wrote:

It might be worthwhile to take @huixie90 's [suggestion from the earlier PR](<https://github.com/llvm/llvm-project/pull/69673#discussion_r1458805792>) into account to add another "`std::in_place_t` layer" to `__expected_base`'s constructors so there is no chance of its `template <class... _Args> constexpr explicit __expected_base(_Args&&...)` constructor "competing" with the compiler generated copy and move operators. Right now, it doesn't really matter as `__expected_base` is neither copied nor moved, but this would change with this PR.

https://github.com/llvm/llvm-project/pull/74768