[libc++] Reland LWG2921 and LWG2976 (#107960)
They were originally implemented in d42db7e083ee0 but reverted later in
a2f3c63282330be0.
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
