Both Clang and GCC diagnose invalid calls to `__builtin_launder`, which
causes duplicate diagnostics when using `std::launder` in an invalid
way. While the diagnostic message for the builtin isn't perferct, it's
definitely good enough to understand the problem and adding our own
diagnostic doesn't really make things any clearer. Because of that, this
patch simply removes the `static_assert`s and lets the compiler handle
diagnosing incorrect arguments instead. This not only simplifies our
implementation, but also improves compile times a bit, since we avoid
instantiating some type traits.
This disentangles the code which previously had a mix of many #ifdefs, a
non-versioned namespace and a versioned namespace. It also makes it
clearer which parts of <new> are implemented on Windows by including <new.h>.
