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
This includes an experimental workaround for
LWG3664 "LWG3392 broke std::ranges::distance(a, a+3)",
but the workaround may be incomplete, I'm not sure.
This should be re-audited when LWG3664 is actually adopted,
to see if we need to change anything about our implementation.
See also https://github.com/microsoft/STL/pull/2500
Differential Revision: https://reviews.llvm.org/D117940
This will detect if someone writes `inline auto cpo =` instead of
`inline constexpr auto cpo =`. I don't know how that'd be possible,
but it's easy to test, so let's test it.
As prefigured in the comments on D115315.
This gives us one unified style for all niebloids,
and also simplifies the modulemap.
Differential Revision: https://reviews.llvm.org/D116570