libstdc++: Add missing static_assert to std::expected<void,E>::value()&&

libstdc++: Add missing static_assert to std::expected<void,E>::value()&&

The r15-2326-gea435261ad58ea change missed a static_assert for
is_move_constructible_v in expected<cv void, E>::value()&&. When
exceptions are enabled, the program is ill-formed if the error type is
not move constructible, because we can't construct the
std::bad_expected_access. But prior to r15-7856-gd87c0d5443ba86, using
-fno-exceptions meant that we never constructed an exception, so didn't
need to copy/move the error value.

So that we don't rely on the r15-7856-gd87c0d5443ba86 change to the
_GLIBCXX_THROW_OR_ABORT macro to consistently enforce the Mandates:
conditions whether exceptions are enabled or not, we should check the
requirement explicitly.

This adds the missing static_assert. It also adds a test that verifies
the Mandates: conditions added by LWG 3843 and 3490 are enforced even
with -fno-exceptions.

libstdc++-v3/ChangeLog:

	* include/std/expected (expected<cv void,E>::value()&&):
	Add missing static_assert for LWG 3940.
	* testsuite/20_util/expected/lwg3843.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>