<optional>: Relational and equality operators are constrained before C++26

[frederick-vs-ja]

MSVC STL constrains some operator==, operator< etc. for optional in C++17~23 modes, while the standard doesn't constrain them until C++26 (via WG21-P2944R3).

Should we

• just remove the SFINAE constraints before C++26, or
• backport the constraints in C++26 added via WG21-P2944R3 and following-up LWG issues, but only for optional, or
• backport all constraints added via WG21-P2944R3, WG21-P3379R0 (for consistency), and following-up LWG issues?

[davidmrdavid]

We discussed this in the maintainer's meeting - we think we 'want to be a little daring' here and backport these to C++17~23 as the constraints seem desirable: these feel like bugfixes.

The one thing that would make us backtrack on these is if these break our "RWC" / "real world code" tests to a notable degree. This is always a risk in any change made to the STL, so that shouldn't block this work.