As the PR points out, we aren't qualifying calls to __ref_cast, and
have 'constexpr' on function templates that can never be usable in
constant expressions.

This fixes it, and also simplifies __variant::__erased_dtor by using
std::_Destroy, although that requires including quite a lot more code,
for iterator_traits and allocator_traits.  If that matters (probably
not) then <bits/stl_construct.h> could be split up to move _Construct
and _Destroy to a new <bits/stl_construct_base.h>.  Or maybe I should
just leave __erased_dtor alone (apart from qualifying the __ref_cast
call).

Anybody feel strongly either way?

	PR libstdc++/80939
	* include/std/variant (__erased_ctor, __erased_assign, __erased_swap)
	(__erased_hash): Remove constexpr specifier and qualify calls to
	__ref_cast.
	(__erased_dtor): Remove constexpr specifier and use _Destroy.