Found while running libc++'s test suite with MSVC's STL.

• I've filed LWG-4021 "mdspan::is_always_meow() should be noexcept" and implemented this in libc++'s product and test code.
• Use LIBCPP_STATIC_ASSERT to avoid issues with noexcept strengthening in MSVC's STL.
  • As permitted by the Standard, MSVC's STL conditionally strengthens mdspan construction/is_meow/stride and elements_view iterator base() &&, and always strengthens basic_stringbuf swap.
  • In mdspan/properties.pass.cpp, this also upgrades runtime asserts to static_asserts.
• Improvement: Upgrade assert to static_assert when inspecting the noexceptness of std::ranges::iter_move. (These !noexcept tests weren't causing issues for MSVC's STL, so I didn't change them to be libc++-specific.)