[libc++][spaceship] P1612R2: Removed `operator!=` from `bitset`

[H-G-Hristov]

Summary

Implements parts of P1612R2:

• Removed operator!= from bitset

[Mordante]

In general LGTM, but I would like to land D152297 first and directly update the status page.

[H-G-Hristov]

Addressed comments

[H-G-Hristov]

In D152611#4411167, @Mordante wrote:

In general LGTM, but I would like to land D152297 first and directly update the status page.

Thank you for the review!

libcxx/include/bitset

764–765

constexpr since C++23 but removed in C++20 - Should I remove the macro?

[Mordante]

LGTM modulo some nits.

libcxx/include/bitset

82

It's a bit weird to become constexpr when it's removed. Please remove the constexpr here.

764–765

Yes please.

[H-G-Hristov]

Removed macro