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
moves tests into directories matching their stable names so that the
tests can reflect the concept name
Differential Revision: https://reviews.llvm.org/D99104