[Bug libstdc++/89022] Implement P0202R3 - C++20 Constexpr Modifiers to Functions in <algorithm> and <utility> Headers.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89022

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to emsr from comment #2)
> I think we're done.
> The __cpp_lib_constexpr may not do anything or may not be in the newest
> drafts anymore.  We should probably kill it.  I was very confused as people
> were going back and forth about constexpr lib macros.

Yeah, it's pretty confusing!

> I still owe the patch for misc. constexpr bits.  That's not really this PR
> though (<algorithm>.  It's stuff like pair ctors, insert iterators, etc.

I think I did std::pair in r276154 (it was needed for Debug Mode which uses
std::pair in some algo checks).

I didn't touch the iterators though.