All supported compilers that support C++20 now support concepts. So, remove
`_LIB_LIBCPP_HAS_NO_CONCEPTS` in favor of `_LIBCPP_STD_VER > 17`. Similarly in
the tests, remove `// UNSUPPORTED: libcpp-no-concepts`.
Differential Revision: https://reviews.llvm.org/D121528
... from testing with MSVC's STL. Mostly truncation warnings and variables that are only used in `LIBCPP_ASSERT`.
Differential Revision: https://reviews.llvm.org/D116878
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
Joe Loser