[libcxx] fixes `common_reference` requirement for `swappable_with`

LWG3175 identifies that the `common_reference` requirement for
`swappable_with` is over-constraining and doesn't need to concern itself
with cv- or reference qualifiers.

Differential Revision: https://reviews.llvm.org/D99817

GitOrigin-RevId: cedd07df5136ba446be23dbd2ca7d29d7b82cd60
3 files changed