[libc++] type_traits: fix short-circuiting in std::conjunction.

Replace the two-level implementation with a simpler one that directly subclasses
the predicates, avoiding the instantiation of the template to get the `type`
member in a situation where we should short-circuit. This prevents incorrect
diagnostics when the instantiated predicate contains a static assertion.

Add a test case that reproduced the previous problem. The existing test case
involving `HasNoValue` didn't catch the problem because `HasNoValue` was in the
final position. The bug comes up when the predicate that shouldn't be
instantiated is after the short-circuit position but there is more to follow,
because then `__conjunction_impl<False, BadPredicate, ...>` instantiates
`__conjunction_impl<BadPredicate, ...>` (in order to obtain its `type` member),
which in turn instantiates `BadPredicate` in order to obtain its `value` member.

In contrast the new implementation doesn't recurse in instantiation any further
than it needs to, because i...