[Bug libstdc++/77537] pair constructors do not properly SFINAE

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77537

--- Comment #4 from Casey Carter <Casey at Carter dot net> ---
Any chance of applying this to 6-branch as well? This is breaking both range-v3
and cmcstl2 in a nasty and hard-to-workaround way.