P3836R2: Make `optional<T&>` trivially copyable

[frederick-vs-ja]

Link: https://wg21.link/P3836R2

[frederick-vs-ja]

I was surprised by the fact that this wasn't implemented in #155202 (Godbolt link).

Looks like that we're not yet selecting correct base classes. CC @smallp-o-p.

[smallp-o-p]

I'll work on this.