[libc++] P2255R2: Changes to Standard library components - `tuple`

[Zingam]

• tuple
  • 20.5.3.1 [tuple.cnstr]
  • 20.5.5 [tuple.apply]

[frederick-vs-ja]

I found that libc++ is currently static_assert-ing problematic reference binding cases for tuple. Should we remove the checks in old modes?

If we want to keep (non-Standard) checks in old modes, perhaps it's better to also backport the deletion of constructors.