[libcxx-commits] [libcxx] [libc++] Reland LWG2921 and LWG2976 (PR #107960)

[Hristo Hristov via libcxx-commits]

Zingam wrote:

> They were originally implemented in [d42db7e](https://github.com/llvm/llvm-project/commit/d42db7e083ee0edda327782aa122ad6474c126d1) but reverted later in [a2f3c63](https://github.com/llvm/llvm-project/commit/a2f3c63282330be0226158cdfdc27c4c2aef9cdc).
> 
> This PR implement both LWG issues again, guarding the removed functions with `_LIBCPP_STD_VER <= 14`, because they should be treated as patches for P0302R1 which was adopted for C++17.
> 
> Fixes #103598. Fixes #103755.

https://github.com/llvm/llvm-project/issues/103598#issuecomment-2339502922

Should these issues be reopened first before landing?

https://github.com/llvm/llvm-project/pull/107960