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

[Jan Kokemüller via llvm-commits]

jiixyj wrote:


> The only downside I see is that `expected`'s padding can never be used by another object. In other words, `sizeof(expected) == datasizeof(expected)` is always true.

So right now, this test fails:

https://github.com/jiixyj/llvm-project/blob/5691b27a2dbe8ed49a328c0cd0619d5af4b930a1/libcxx/test/libcxx/utilities/expected/expected.expected/no_unique_address.compile.pass.cpp#L53

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